Going with manual brakes on a 2800lb~ car, 11" disc front, 9-10"drums in the rear, are these really worth a damn? Anyone running one of these valves? and how much does it help keep pressure in the line(less energy needed to pump them up, so ive heard)
YES they are worth a damn! IMHO They serve to LIMIT pedal pretravel by keeping a wee tiny bit of drag on the brakes. Different setups and brakes require different valve pressures. For instance if your M/C is mounted up high (higher than your calipers) you could likely getaway with NOT running a RPV on the DISCs - but IF your master is at or below the calipers then running a 2# RPV is a good idea. Your drum brakes are less efficient than your calipers (due to the return springs in part) so you need a heavier RPV there - I think 10# RPV is typical there. Keep in mind that RPV's are NOT a NEW concept - the factory has been using them for several decades. I don't know when they started them, but I just had my '47 M/C apart and there's one in there.
You must have: - 10 psi valve on the drum circuit, always. - 2 psi valve on the disc circuit if MC is mounted lower than calipers. These aren't items you can "skimp out" on if you want a safe brake system.
I'd like someone to tell me exactly why they are a MUST. In the Fairlane I have: mustang mastercylinder mounted on the firewall. Disc brakes on the front. Stock drums on the rear (8"ford rearend) In line going to the rear, I installed a wilwood proportioning valve. ...and that is it. No power assist or anything. NO Residual Pressure Valves. The system seems to work fine. Way better than what was there stock. Am I missing something?
Yes, you're missing something. Your mustang master cylinder has a built in residual valve for the drum rear brakes. And a firewall mounted master usually doesn't need one for the front disc brakes. as was mentioned, residual pressure valve have been part of most master cylinders since hydraulic brakes came into use in the 30's.
I think a lot of people new to building and setting up things like brakes (like me)...see those shiny RPV valves from Wilwood in the catalogs and think they are something that they need to get. Hell, in the Speedway catalog there is a diagram showing clearly that you NEED those RPV valves... But there are indeed RPVs already in mastercylinders from the factory. Sooooo... this means that most people DO NOT need those aftermarket wilwood RPV valves? Seems that way to me. especially if you have a firewall mounted mastercylinder. I guess I could see using a 2# RPV valve for the front discs IF you have the mastercylinder under the floor..... Right?
Tingler - I think you've hit the nail on the head!!! Yep....there's no reason to have to go out and buy some high priced aftermarket RPV -when the factory ones have been working perfect for eons. To add to your comment about factory installed RPV's - I recently discovered that the M/C wasn't the ONLY place that the factory put those!! I found that they ALSO put them into the REAR brake "T" fitting mounted on the rear end housing. I don't know who did this, but in a recent sifting through my brake fittings (stuff saved from various cars over the years) I discovered that I had one where this was the case. Yep! Yep!! .....Tingler you say you're NEW to setting up brakes - sounds to me you're getting a really good handle on them RPV's quick!!!
Tingler, I'm not claiming to be an expert on brakes or anything, but to just to ADD to the reason why we need RPV's and just incase you or anyone else is still wondering (my understanding is this): AS the name implies Residual Pressure Valve - is there to maintain residual pressure in your brake system. Residual pressure means that after removing your foot from the brake pedal there is still SOME pressure in the system - this is a result of the RPV. We've always learned that dragging brakes are bad - right??? Well in a perfect world this is true - however in reality there are things that prevent the ideal. Things like front rotor run out - as the rotor runs out (wobbles) it can push back the pads - remember how LARGE your dics brak pistons are??? Well if you have an excessive gap between the pads and the rotor the end result is that you will use a fair amount of your pedal travel in closing the gap - (keep in mind your m/c piston is SMALL diameter - for mechanical advantage - this means you ALREADY have to push it in MORE than your caliper piston moves out) so you may find yourself having to "pump up" the brakes every time you went to use them if you had no RPV. Also IF your m/c is mounted somewhat HIGHER than your calipers the result is a naturally acting residual pressure (gravity) so no additional vavle is needed. Drum brakes have heavy return springs built into them. This is a result of wanting to prevent shoe drag. Drum brake shoes by design have a certain "power assist" affect due to the mechanics (servo action) and as a result having them drag even a little would generate a bunch of drag even moreso than you might think. This may sound contradictory (having a higher RPV valve rating here) but the saving grace is that that return springs are pretty heavy and therefore you don't get the full effect to the shoes - instead we are trying to maintain a minimal air gap - as a result the RPV and return springs need to be matched - we take this for granted with comments like "10# RPV's for drum brakes - period" well - like anything the 10# valve works a majority of the time - but it ISN'T magic - there are cases where it WON'T WORK. The OTHER reason why we need a higher RPV in a drum brake is because when we retract the piston (brake release) in the absence of pressure the lip would allow us to suck in air EVERY time we released the brakes!!!! Maintaining a certain amount of line pressure PREVENTS this. So bottom line it is FAR MORE than just minimizing pedal travel in a disc system (although that'd be reason enough) but is even MORE important in a Drum System It's all a balancing act of sorts. And I am sure I will get major GEEK points for yacking so much - oh well - shit happens. I hope I added to your understanding of RPV's - because in my mind understanding a system alows us to more easily fix it when it's broke.
I'd just like to add that if you run a pressure switch to turn on your brake lights the residual press valve will likely be enough pressure to keep the lights on, so plumb it before the valve. Or if you have a rpv in the M/C then use a lever type switch.
quick way to tell if your master cyl. has a residual valve- I unbend a paper clip, drop it in each port. If there is an rpv it will stop at a quarter inch or so, if none, it will drop in considerably farther. DON'T PUSH it in, you can puncture the diaphragm in the rpv.
Sorry HemiRambler, but I'm gonna have to disagree with this part of your post. The reason the residual pressure valves are there in drum systems is for your second reason, e.g. to keep the air out the system when you let the brakes off. They are not there to hold the shoes in some semi-retracted position, floating just clear of contact with the drums. Think about it this way - if the RPV did actually hold enough pressure to stop the return springs pulling the shoes back to their at rest position, you would never have to adjust drum brakes. Or to put it another way, the reason that you have to adjust drum brakes is to keep the shoes close to the drum by mechanical means, because the springs do pull them all the way back.
Sorry SimonSez, but I will respectfully have to disagree with you. Case in point: I just converted the Master cylinder in my '47 Ford to a Modern dual piston type with power assist. I had all the parts I needed EXCEPT I am still waiting for my RPV's to arrive. Mix curiosity and impatience together and I decided to plumb it up (temporarily) WITHOUT the RPV's. SO given YOUR explanation it ought to work pretty much like "normal" and the only "problem" is risking sucking in air upon piston retraction. My explanation would result in a bunch of extra pedal travel as the shoes (or better stated the pistons) are now retracted. Guess which way it is??? Yep - first 1/2 of the pedal travel results in no actual braking - there is enough gap that with the first 1/2 of the pedal travel - all I am doing is removing it. And before you suggest I am sucking in air - I initially blead the crap out of it - bench blead the m/c - the whole nine yards. It acts EXACTLY the same after driving it. It doesn't pump up (suggesting there is no air in the lines) nope - just a bunch of wasted pedal travel. For the record - I expected this - I was just so excited about the whole conversion I wanted to see exactly how much the lack of RPV's would matter. They "matter" ALOT!!!!!! So given this I am gonna continue to respectfully disagree with you. Your logic about the "never having to adjust the shoes" - if I was right doesn't hold water in my head since the shoes are basically pivoting about the lower adjuster - retracting at the top (piston end) would result in excessive pedal travel - just like I have. "The reason to adjust the shoes is to keep the shoes close to the drums" - I AGREE with your statement there , but it doesn't necessarily keep the PISTON SIDE CLOSE (well on my '47 it does)- what that DOESN'T do is keep the piston CLOSE to the shoe - in my case. And maybe THIS is where we are tripping over one another - I believe I previously have said "shoe to drum" and what I really mean is the "shoe to piston" and or "shoe to drum"- bad choice of words I suppose. In any event - the associated "gap" is REAL and causes alot of wasted motion. This is a BIG part of the benefit of the RPV. The 'air suckign in' part seems very secondary to me as I have yet to experience it - but I will NOT drive this liek this long enough to find out as my RPV's are due in any day now.
To add a couple of things to the mix: According to some participants on other hot rod boards - and this one as well - Wilwood RPV's have been having high failure rates in the last few years so it may be a good idea to spring for a different brand here. As far as proportioning valves go, if you're running big and littles with considerable disparity in diameter - as on a typical highboy roadster or coupe - more than likely you won't need a propo valve. I have one on my 32 and find it works best cranked all the way off. The 31 on 32 rails roadster has big & littles, but the disparity in tire diameter is not as great as the 32 has. I'm not going to install a propo valve initially, but will plumb the system so that one is easily added. Fwiw - tire diameters on both cars are: 32 = 24" and 30 1/2". 31 = 25" and 28". One of the mags interviewed one of the bigger aftermarket hot rod brake suppliers and they also recommended trying a no-propo system to see how it did if your car was equipped with big & littles. What's happening is that the larger rear tire has more leverage on the rear brakes and requires a higher pressure before lockup and things tend to balance out well on the typical disc front, drum rear car. One thing I am gonna try is one of the hold-off (metering?) or whatever the heck you call it valves that go in the front brake line. They hold the front disc brakes back until a specific pressure is attained in the rear brake's hydraulic lines and then let pressure bleed into the front brakes. Again, on a disc/drum setup. It's an aid to straighter stopping from what I understand.
For the last ten years or so manufacturers did not a have a residual pressure valve in the master cylinders (dual setup). The earliers ones did. Residual pressure was done by the combination valve (metering and residual pressure) so you better be sure the Mustang has a residual pressure valve for the rear.
That's cool - we don't have to all agree I think there is one way to check for sure, but I haven't ever tried it. Once you have the RPV's plumbed in and the system all bled up and working properly take a drum off and see where the shoes are sitting. Then crack the bleed nipple to let the 10psi out and see if the shoes retract any further. cheers, Simon
I think you're supposed to put it in the line to the rear brakes. The explanation I heard was that you want your front brakes to catch first. If you have a factory proportioning valve, you don't need an adjustable. Anybody got the bottom line here? My front (M2 setup with 11" Ford Discs, 2# RPV) brakes don't grab real good. I have a suspicion that it's the braided stainless flex lines, because I've never used them before, and I'e never had this problem before. I thought it was just my rears grabbing too soon, so I installed an adjustable proportioning valve, but it didn't do much for me.
As far as I know I have always been told that an RPV is used only if your master cylinder is lower than your wheel cylinders or brake calipers, the reason being that gravity will create enough pressure to keep the fluid from draining back to the master cylinder, as far as sucking air in the system I don't see how it's possible to pull air into the sytem unless you have a leak already or you have the MC mounted low and it's so low on fluid it pulls through the MC . Marc (stickylifter) I have heard that if you don't have a MC with enough volume for front discs that it pretty much negates the affect a proportioning valve has because a PV only restricts the flow (pressure) to the rear and if you don't have enough volume to the front in the first place it won't make any difference.
Yeah, I think that's my prob. Either the stainless flex lines are restricting the flow, or the front half of the MC is just too small. The back brakes work like gangbusters. It's not like I have no brakes (you've ridden with me-you know) but I feel they are at 75% of potential.
Ambman, I think you've been told "wrong". The should always be an RPV in the system - assuming we're talking drum brakes. When the master is ABOVE the calipers then theres' already a little pressure in there from simple gravity. When the installation doesn't allow this (master below the calipers) then a small 2psi valve is required. Drum brakes can always benefit from having the higher (typically a 10 psi) valve.The thing about it is that alot of master cylinders have them built in so some poeple don't even realize they have 'em. They are also built into the "combination" valves again - so some poeple don't realize they're running one and go on to tell people they aren't necessary. As far as "sucking air" - YES it can happen - although I ran my truck a couple weeks without one and it didn't. Given that one might be inclined to say they DO NOT perform THAT function. NOPE!!! I think that just becuase ONE limited test doesn't show a problem DOESN'T make it gospel!!! My seals were BRAND NEW - bores just honed - I EVEN misadjusted the brakes so the wheel cylinders would have to travel TWICE as far as normal - in an effort to try and INDUCE some "sucking air" - I couldn't get it to happen. What does this PROVE??? Well nothing really. Under different circumstances I may have had different results. For instance what if I had manual brakes - with worn seals - and a STIFF pedal return spring with the "eyelet" type push rod - forcing the piston back HARD - well maybe THAT would eb enough to "suck" some air in past the seals. This happens at the wheel cylinders -the factory dealt with this in TWO ways - Residual Pressure and a seal reinforcement washer (sorry - I don't know the correct term) you may have notcied them when changing wheel cylinder seals. Your "proportioning valve arguement" makes some sense, BUT!!!! Keep in mind your Proportioning Valve isn't some useless option - if so the factory would have scrapped them a long time ago - they got rid of grease fittings - saved them pennies - so be reassured they use parts because they NEED them for OPTIMUM performance. Sure your system will work without it - but it won't work as GOOD. Master cylinder volume - good point but the volume really has MORE to do with SERVICE LIFE than each application of the brakes. To prove that go look up the M/C PISTON diameters - you'll see the disck brake sizes overlap drum brake sizes as well. The increased volume is because as you noted the disk brake pistons are LARGE - when the pad wears ALOT of fluid is needed to fill the void. Nothing more involved than that (argueably). If your braking is less than it "ought to be" then - look for the basics first. Lines collapse - pads get contaminated - proportioning valve is BAD -fluid becomes saturated with water - SILICON FLUID is COMPRESSABLE - way more than DOT 3 or 5 is - it also DOESN'T absorb water allowing it to form localized puddles - these localized puddles can heat and boil turning to vapor - resulting in spongy pedal or worse!!! The "regular" brake fluid absorbes moisture and dissipates it throughout out - preventing the "vapor release" as a result of boiling. Instead it LOWERS the overall boiling point of the fluid as it dillutes it. Resaon to change your fluid every few years. Keep in mind that in EITHER case - Silicon or Dot 3/5 - neither CAUSES the water to get in there - it's not magically drawn in. It happens as aa result of "normal" system failures - seals not sealing etc etc, the problem is what happens once it gets in there - and sooner or later it will. Sure flexing lines can be a probelm too, but don't forget to check EVERYTHING and not get hung up on a hunch. Hunchs are good but if they cloud our judgement they are the worst. Start from the basics - be methodical. Seems my "luck" is always better when I take that approach.
As long as we're on the subject... I just installed a '67 Mustang dual master cylinder (drum/drum) under the floor on my RPU with '40 ford drums all around. Do I need RPVs or not? Thanks, Rich
My question about the "sucking air into the system when the brakes are released" is where the hell in a CLOSED brake system is air gonna get sucked in??? am i missing something here?
"CLOSED" system is what you are HOPING for. When this happens it occurs on wheel cylinder piston RETRACTION. When the piston retracts the lip seal is at it's weakest (most inefficient) point because the pressure is gone and instead is actually negative since you are now POTENTIALLY PULLING back the seal. It is at THIS POINT when your system CAN become "OPEN" and suck in air. Liek I said before - this is what people who are smarter than me tell me CAN happen. I could NOT get it to happen on my truck - and I did try. But I also limited my effort to a weeks worth of limited driving. Again - I SUSPECT that a given set of circumstances make this more or less apt to happen. Remember the factory designs these systems to operate at EXTREME conditions. Parts can be in various states of condition - from new to 1/2 worn out or worse. Temperature in a brake system can go from far below freezing to SOAR above boiling in an instant. What exact circumstances causes this - I dunno - I ain't no expert THAT I can guarantee you. We ASSUME the system is closed - IF this were truely the case it would NEVER RUST inside because MOISTURE would never get in there. We all know that they do rust - why?? 'cause it ain't perfect. Go look at your M/C piston see the little 'valleys' in the seal - what do you think those are for????
Hey guys, You all need to throw away the California carrots you been smoking and go here>>>>>> http://www.mpbrakes.com/mpfaqvalving.htm FAQ sheet on any valve you care to mention. DADSEH
Wow THAT'S great - the volumes and volumes of useless info boiled down to a single page of COMPELTE ANSWERS and total understanding - wow - I'm gonna memorize THAT page and now tell everyone I am a brake genius!!!! Please STOP me!!!!!!
I have dealt with Inline Tube when plumbing my 66 Plymouth and that's the information I got from them, I may not have explained it as well as the guys there but all I can say is if you need to talk to someone who are "experts in the field" they are more than happy to help. On a side note I have known of people who have converted to disc brakes and not changed the MC (I wouldn't recommend this) and just put in a proportioning valve and it acts the same way Marc described his car.
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