I'm in process of converting to manual front disc brakes on my 39 Plymouth pickup and have selected a one inch bore master cylinder with a common reservoir from a 1995-2001 Jeep Cherokee which by most accounts should adapt easily and work well for my application. My question is: Does it make any difference which MC outlet ports are plumbed to the front discs or to rear drum brakes? Seems to me it may not matter, but perhaps there are internal differences? Any knowledge would be greatly welcomed.
Biggest reservoir goes to front disc, smallest to rear. So if front reservoir us biggest that means front fitting goes to disc.
The newer clear fluid reservoirs are divided internally, just not as visual as the earlier metal reservoirs. You really need to know the designed front/rear plumbing for that master, because it does make a difference.
Usually on a disc/drum dual master the front disc brake lines are 3/16 and the drum are 1/4 . This is because the front disc like high pressure low volume and the drum like lower pressure more volume. Of course these lines generally go directly into the proportioning valve where usually the lines stay smaller diameter leaving the proportioning valve for the front discs and larger for the rear drums. Also the bore of the mastercylinder has to play well with the proportioning valve and the bore size of the front calipers and rear wheel cylinders to work properly. There is a lot more to it than just bolting a master on and hoping it all works together. So usually but maybe not always the smaller master cylinder brake line port is meant for front disc brakes.
Generally, most lines are 3/16" on a disc/drum system. It's the line fittings that are sized differently to prevent them from being incorrectly switched during initial assembly and any service work.
like tri power said, that bore size plays a big part! usualy that 1 inch bore gives a hard pedal on a disc drum setup.
I agree the whole car can be done in 3/16 line and many cars are this way. Hydraulic fluid doesn’t know what size line it’s flowing through. But usually masters have 2 different sized ports and generally the smaller one is furthest from the firewall and usually the smallest port is meant for the front disc brakes. And many cars keep the entire vehicle with 2 different sized lines for the whole system- disc being the smaller diameter It would be interesting to see pictures of the new master cylinder ports and the old master cylinder fittings??
This is partially correct except the line size part, doesn’t matter what size the line is as far as pressure. It’s the master bore size and input force that determines pressures, line size has nothing to do with it. It’s true that most relatively modern drum brakes are self energizing and disc brakes rely strictly on clamping force to stop however unless it is a stepped bore master than they both see the same pressure, the clamping force is determined by pressure and size of output piston. If it is not a stepped bore master then as far as function it makes no difference which reservoir is plumbed to which end, the difference in reservoir size is the volume of fluid required to accommodate the pistons moving out as pads wear. Sent from my iPhone using H.A.M.B.
Here's a photo. Large (12mm) fitting is towards front and small (10mm) fitting towards rear. The orifices are the same size.
Take the cap off and look inside - should be that simple! You've looking for a divider / wall which will have a height somewhere in the region of the 'low' level, or at least lower than the 'max' level. Chris
Learning the OE pri./sec. plumbing is what I meant. That style master was designed to be mounted angled up to level the cap, and it looks like the primary (closest to the mounting flange) section of the reservoir is larger than the secondary, so the primary is probably the front disc circuit. But, I would not use that master mounted level because of the angled reservoir. There are many older style disc/drum 1" masters that have level and obvious disc/drum fluid reservoirs to choose from.
If you look at the reservoir in my avatar picture it was a early 80s elcamino style and is supposed to be angle mounted but the previous owner mounted it level and it looked odd and wouldn’t fill properly. After much research I figured late 90s Chevy s10 master had the same bore as the elcamino master and the fittings even matched up. It was also a direct bolt on to the booster. V8 Bob is exactly right this does look like a angled master and it does look like the orifice closest to the firewall is the front disc circuit - also notice on the master you have shown that the larger part of the reservoir is to the rear. Here is pictures of my new master. Note the thinner 3/16 line is at the rear of my new master but it used to be at the front of my old master...I think. What does the master look like you are replacing and why are you changing it? What did the master you are replacing come off of ?Maybe we could help you find a more suitable one?
Yeah, I am not a fan of the angled and plastic reservoir either, however this master cylinder has been used successfully by quite a few 1930s to 1950s Mopar truck owners. It adapts easily and fits in a tight location between the steering box and the starter. I'm thinking the larger reservoirs on many disc / drum master cylinders are made that way only due to more volume of fluid needed on the front disc side as the pads wear down. I think that would be important for daily drivers whom most don't check fluid often if at all? I was able to adapt a disc / drum split master cylinder (15/16" bore) while I was still running all drum brakes. I did this in preparation for the front disc project. I had immediately had a pedal travel issue and other weird braking things after removing the original single stage master cylinder (1.25" bore dia.). I did not do my homework that time and did not install any residual valves in the system, which I'm now thinking was causing the weird braking and possibly excessive pedal travel? Anyhow, my logic was to up the MC bore diameter slightly to 1" to help with the pedal travel. If my calculations are correct? I should have a decent setup calculating at 70lb leg force with a 7.5 pedal ratio = 525 / .785 (mc bore surface area) = 668.8 psi X 4.9087 (2.5"caliper bore surface area) = 3,282.9 clamping force. Does this sound correct or reasonable?
Yes, your pedal travel was a result of the smaller bore, it will increase pressure but at increased pedal travel. You are also correct about the reservoir size. Sent from my iPhone using H.A.M.B.
I'm am using the same plastic master you pictured off a '98 Cherokee on my '53 Dodge 1/2 ton with disc/drum and a proportioning valve that is plumbed front of master to front of proportioning valve and rear of master to rear of proportioning, then from the rear of the proportioning valve to the front (disc) brakes and from the front of the proportioning valve to the rear (drum) brakes. That is the way it was plumbed on the donor I pulled it from and has worked flawlessly since. Things are kinda tight in there so the proportioning valve is mounted between the master cylinder and the starter. The reservoir is not mounted level.
Beautiful car Tri-power! I'm changing it because after I installed a dual stage master in preparation for my front disc project I immediately experienced excessive pedal travel and the brakes were doing other weird things. Anyhow, the oem lockheed brakes worked really well, but were a pain to adjust and after installing a T5 trans and a mid 80s jeep rear axle along with it's bendix drums brakes I thought I would upgrade the front to disc as well. Seemed a sensible time to get rid of the original single stage master cylinder. Now, I'm not so certain? When I got the junk yard parts I grabbed a master (on bench in photo) off of similar vehicle (mid 80s Jeep Cherokee / Comanche). Being uninformed and a bit overzealous I rebuilt it and made an adapter and shoe horned the thing in there. That's when my troubles began. I'm now thinking the the root of my problems were not having any residual valves in the system which I now know needed them. The mid 80s Jeep master might actually be OK, but the outlet seats are damaged and it's hard to find this one made from aluminum which I would rather have. Being too fussy, I would have to paint a cast iron one. I read where others with old mopar trucks had luck with the 95-2001 Jeep Cherokee master cylinders. I'm sure you guys are correct and probably others mc's that might work just as well without the angled reservoir?
Thanks for reply Oil Soup! Yes, I too have a Combination / proportioning valve from mid 80s Jeeps I grabbed from a donor vehicle. After much reading and total confusion, I've come to the conclusion I'm not going to install it. I'm going to install an aftermarket adjustable proportioning valve in the rear brakes and a 10lb residual as well. Probably install a 2lb residual in the front if I need it. This way I can adjust the bias if I need to. I am perplexed why none of the manufacturers make an adjustable combination valve? One that includes a metering / hold off valve? There must be some reason for this? I know a lot of street rodders install factory style combo valves that were designed for specific vehicles, but I suspect many may have rude awakenings in panic stop situations? I'm curious if you have had any issues at all after installing your master and disc upgrades? Are you using a vacuum assist? Which company's conversion kit did you use?
I'm confused by pedal terminology. Does "hard pedal" mean a bad thing? Like difficult to stop or is it a good thing like opposite of a soft pedal which sounds spongy to me?
Thanks Tri-power! I looked up the late 90s Chevy S10 master cylinders you suggested and it looks like it may be a better choice for me? I'm wondering if you might know the center to center distance of the mounting holes or studs? If it is similar to mine, I may even be able to reuse the adapter flange I made for my first attempt.
I just noticed that your master cylinder lines are exiting towards the left of your car. The S10 master cylinders I see on Rockauto are all exiting on the right side? I must be missing something?
I have some thoughts for you. No.1: when the master is above the calipers on disc brakes, you should not need residual valves. The 10lb on a drum is to keep the springs from completely retracting the shoes after release. No.2: for most, the adjustable valve only on the rear will work very well. The factory valves many times are not proportioning valves but just a sensor location for a brake light. No.3: you may find that you need a 7/8" bore master to have adequate pressure to have modern braking. Many will dispute this but go check and see what most power brake masters have. You know you can't equal that pressure with your foot in normal applications. If you use a Ford master such as late 60's to early 70's, you can find an assortment of bore sizes and lines coming out from either side. Good luck with your rod.
The 10 lb on drums is to keep cup seals in the wheel cylinders from collapsing and allowing air in upon brake release. Has nothing to do with the shoes or return springs Sent from my iPhone using H.A.M.B.
Steve Lowe the part number on the master in my picture is mc390572 it is a Sonoma - s10 master but the ports are on the opposite side under this part number. When I wanted to find a different master for my car and knowing it came off of a g-body el Camino I contacted some of the guys on the Malibu websites they used this master on the Malibu’s they owned as a upgrade( bigger reservoir) There is a guy called malibudaves manual brakes.com who has kits and helpful information on manual brake stuff. It’s been awhile since I changed this master but I think the ports are on the other side of the master is meant for a s10 model that is right hand drive Figuring out what other people have used in the same circumstance as you is often the cheapest and easiest route. I bolted on a few different masters I researched and thought they would work and they did not.
Steve Lowe the center to center distance on the mounting holes of this master are 3 3/8 or 85mm ( I am Canadian)
That looks like a stepped bore M/C [The outside diameter of the machined area appears to be larger than the casting] If it is a stepped bore, the port nearest the firewall/mounting flange is for the front brakes. Stepped bore M/C's are designed for low drag calipers. The large bore is used to take up the initial pad clearances [low pressure], then pressures are equalized and it behaves like a normal dual M/C
I'm curious if you have had any issues at all after installing your master and disc upgrades? Are you using avacuum assist? Which company's conversion kit did you use? I'm using RustyHope disc brake conversion with no vacuum assist and have had no issues. Of course I don't have abs but the brakes work fine, no pulling, stops straight.
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