Ongoing brake problems, Can you help?

parts:

frame mounted setup
7" dual diaphram booster
Dual resevoir master cylinder (equal size res's)
Disc/Drum GM combination valve


I've been battling getting my brakes dialed in for what seems like months. I have no good pedal pressure. This weekend I guess I will pull the master cylinder and try and bench bleed and test it one last time before I buy ANOTHER new one.

Does it really matter what resevoir is plummed to what port on the combination valve? Both resevoirs hold the same volume. Will they feed at different speeds. On mine I plummed the front port on the combo valve to the resevoir closest to the booster. In most diagrams I see the front port on the combo valve plummed to the rear resevoir on the MC. Is there really any difference in how each resevoir will feed the system? Switching those around is the only thing I have not tried to get my brake problems shaken out.

Can you help?

 

Yes it does the caliper uses more fluid volume...I believe the rear reservoir is for the calipers? maybe some others will chime in.

 

Certainly it matters. HRP

 

Sounds like you have them backwards?

 

I seem to recall a rule of thumb that the rear outlet on the m/c supplied the front brakes and the front outlet supplied the rear brakes. Thinking that this setup provided braking force first to the front wheels and then, progressively, to the rear wheels.

I'd guess that if the fluid reservoirs are the same capacity the cylinder might have been intended for use with 4 wheel drum brakes or 4 wheel discs, but not a drum/disc combination.

http://www.earlycuda.org/tech/dualmaster.htm

 

It seems like many of the aftermarket disc/drum kits from numorous mfg's have equal volume reservoirs, but I can't find any details about these units being front or rear specific. All the research on other forums I have done shows conflicting opinions from "so called" experts saying it does and it does not matter. I guess if the bench bleed does not work this weekend I will consider bending some new jumper brake lines to run the reservoirs the other way around.

So you want the rear brakes to grab sooner than the fronts in a disc/drum application? I would have thought the opposite, but if the res closest to the booster is for the rear brakes, I would think it gets pressure first.

 

Brake application timing will be similar to what I know in big truck air brake. The most rear applies first and goes progressively later toward the front of the vehicle, and release is in digression as the same. This keeps the vehicle stopping in a pattern that is less likely to cause a jackknife in a big truck.
I know that in cars and pickups, your braking power is greater in the front brakes, but you have to apply the rear first to keep it in controlled direction. Mismatch in parts will cause other problems as traction/road conditions will too.
First stage to rear of vehicle, closest to pedal application point.

 

Claymart>>>I'd guess that if the fluid reservoirs are the same capacity the cylinder might have been intended for use with 4 wheel drum brakes or 4 wheel discs, but not a drum/disc combination.>>>

I'd guess the same. So, RatPin, are your actual brakes all drum, all disk or drum/disk combo? I don't think you mentioned this?

Jack E/NJ

 

So what have you been using for a guide or instructions ?
What do they say ? You seem frustrated and lost.

Now most ( meaning not all) but most equal size resivours are for disk disk or drum drum brake set ups. It most certainly takes more fluid to operate the disks than the drums, so you should be able to tell which pushes more fluid in your bench bleed. ( please read the directions supplied with the master on how to do this, they are not all the same )

Another thing to be aware of it that your fittings can and will suck air with out leaking if full pressure has not been achieved. Brake lines that run up and over can trap air. Brakes that are out of adjustment do not give good pedal feed back. All systems with master mounted under the floor need residual pressure valves.

Some more details on your components will help
http://mbmbrakeboosters.com/index.php?option=com_content&view=article&id=10&Itemid=17

Take a peek at that

 

The kit I'm using is part #BR2918 It's half way down this page:

http://www.americanclassic.com/asp/dyncatv2.asp?which=amclsall&p1=initial&p2=Brake+Power+Boosters+and+Pedal+Kits&iyear=1954&ytype=to&exyear==

This is an MBM kit.

Disc/drum is what I'm running on my truck.

 

so, what car are you working on and what brake system are you working on, as asked above? Your Public Profile tells us nothing about you or your ride. post some pics of ride and M/C, etc. 7" booster is not for all vehicles.

 

It's a '54 International pickup. Frame up build as seen here: http://www.jalopyjournal.com/forum/showthread.php?t=614359




Mustang II front with GM metric calipers and the stocker rear drums.

 

I ment pictures of the brake system, lines, routing, blah blah
Neat truck !

 

Don't really have any off hand. Pretty basic. I do have the rear line stepping over the frame notch in out back before going to braided lines. I also have the single out line to the fronts splitting right before the crossmember and also going into braided stainless flex lines to calipers. I've had no problems bleeding the lines and get decent pressure out the nipple when pumping the lines to bleed them.

 

I've been battling getting my brakes dialed in for what seems like months. I have no good pedal pressure.


And then



I've had no problems bleeding the lines and get decent pressure out the nipple when pumping the lines to bleed them.



I'm confused now because this is contradictory, maybe some elaboration will lead us closer to the problem

 

Plenty of pressure to drive fluid with bleeder valve open when pumping pedal, close everything up and pedal goes to the floor when pressed. Pump pedal repeatedly and it has fair brake pressure let pedal rests momentarily and it goes to the floor when pressed again.

I had adjusted the booster rod out long enough to a point where the brakes were drivable, (not great, but drivable) but after 15 minutes of driving or so the front brakes began to drag and squeal from what I assume was being caused by the piston not being able to return fully causing pressure to build up. I would adjust that rod in slight increments shortening it and it would go from that to pedal to the floor. At this point I can only think there is air trapped in the MC or the MC is bad.

I'll bench bleed it again and test to see if it holds pressure with all ports plugged.

 

Do you have residual pressure valves in there ?

Lengthening the pushrod to that extent makes the pressure remain like the rpv does but too much and makes the brakes drag .

 

All the GM type I have done rear outlet=front brakes. I am not sure about the porportioning valve you have but I am sure it thier is a right and wong way to plumb it.The front brakes need more fluid.

 

I do not have residual pressure valves in line. I bought them and was then told that that GM combination valve was an all in one and did not require the residual valves.

 

That's most your problem 100% guaranteed
You gotta have them with a low under floor master
Think about your systems , think about what the rpvs do.
Check that link I gave a few posts back.

Ask who told you that where they got that info and why your brakes don't work

 

There is the very very slight possibility that you won't need them on the rears if the master has them already built into the drum circuit. Most likely you'll need a little more and have to add it in.

All drum brakes need a RPV regardless of master location. And most drum drum masters have them built in. Any disk setup with low master needs inline units.

 

Hey thanks, you're right. I found this on that site:


Does that 2 lb residual need to be right off the MC or can I mount it further up on my frame towards the front? At least I still have those valves laying around new in the packages. Now I just need a nice flare tool.

 

You can put them where they fit after the proportioning valve before the T fittings

 

RatPin>>>Plenty of pressure to drive fluid with bleeder valve open when pumping pedal, close everything up and pedal goes to the floor when pressed. Pump pedal repeatedly and it has fair brake pressure let pedal rests momentarily and it goes to the floor when pressed again.>>>

A worn-out cup or corroded cyl walls in the mc are also possibilities.

Jack E/NJ

 

Call Master Power Brakes, 888-351-8781, www.mpbrakes.com or ECI, 860-872-7046, www.ecihotrodbrakes.com. All these guys do is braking systems. The Master Power catalog is a wealth of info!

 

Did you check to see how much vacuum you have ?

 

I have not checked it yet, but noticed when I push the brake pedal the idle raises ever so slightly for just a fraction of a second.

 

We had a similar problem with a late model motor in an old Mercury and we had very poor vacuum, had to add a pump.

 

Just need to get me a good flaring tool now.

Thanks for the help guys! Especially you 31Vicky.

 

Well I'm going to stick my foot in it here, so to speak.

First off, I think I was wrong in my assumption about wanting the front brakes to apply slightly before the rears. I'm pretty well convinced it's the other way 'round.

Now let's make some more assumptions for the sake of argument. Let's assume that:

1. The m/c is the same bore size for the front and rear halves.
2. The m/c has residual valves for both circuits.
3. The car has drums on all four corners.

I've still got it stuck in my brain that the front outlet of the m/c supplies the rear brakes and the rear outlet supplies the front brakes. I'm basing this on the belief that the two pistons in the m/c move independantly of one another with a spring between them. Differences in tension between this spring and the spring at the front of the m/c allow pressure to build first at the front of the m/c applying at least some amount of pressure to the rear brakes before the fronts.

Or am I just remembering some little bit of info about one manufacturer's brake system that wasn't universally accepted by other makers?