rear pressure low in system HELP

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Brad426

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I installed a Right Stuff kit in the car and I have a disc on the rear. 1969 Plymouth Valiant.

I'm not getting enough fluid pressure to the rear disc callipers to have much of an effect. Engine running and hard on the brake peddle the rear tires still turn in drive. I have bled the system several times and there is no air left in the system. I have tried the system with the manual proportioning valve all the way open and all the way close and several settings in between. I think what is happening is that the front brake circuit (rear reservoir) is building up pressure and stopping the master cylinder piston from traveling further in the bore to build up pressure in the rear brake circuit (front reservoir). The line lock valve is not connected. There is no stock proportioning valve in the system.

I'm stumped on this one and need your advice.

Thank you in advance.:prayer:
 

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What you describe "in the master" cannot happen unless the front piston is already bottomed.

Is there ANY possibility that instead of a pressure problem, it's a rear brake problem? They drum? disk? shoe adjustment?
EDIT Missed the fact that you have rear disks EDIT

Might be time to "rig" a pressure gauge so you can be sure it's actually a pressure problem

Otherwise, I'd be "bench bleeding" the master, and if that does not work, tearing the master apart to see what.................

You might also get someone, or a mirror, so you can see fluid action in the master. You should be able to apply the brakes, then see a EDIT small amount of returning fluid from both reservoir ports

Maybe the front piston is missing the return spring, or has a defect, sticking in the bore???
 
I know its confusing.....many aftermarket master cylinders don't contain residual valves like the factory ones did...Also, some people say you only need them if the MC reservoir is below the floor which would allow bleed back from the higher calipers....but it won't hurt to have them on anyway.....The master cylinder you are using there appears to be a disc / drum application.....A disc/disc master cylinder has a longer stroke........The reservoir for the discs is larger due to the fact that as the pads wear the caliper extends more and is taken up with brake fluid....Whereas on Drums brakes the added wear is taken up mechanically with the self adjuster, thus a smaller reservoir.... One cheap fix you can try is to place a residual valve in the rear line somewhere and see if that helps ....the 2 lb. ones are for disc and 10 lb. ones are for drums....Also, I like to run the 2 inlet 2 outlet style proportioning valve so everything is regulated through that one prop valve......Just make sure you get a disc/disc valve....
 
I talked to some shops today and I'm waiting for a return call from Right Stuff. Things are starting to point to the master cylinder. My plan as of today is ask Right Stuff the application of the master cylinder & order a "NEW" one localy and try that. I'll keep you posted.
 
Thank you everyone so far. I'm still scratching my head on this one still. :banghead:

Question: Must you have a Proportioning Valve Disc / Disc regardless of having a manualy adjustable valve like I do? Can you have both? Should you only have one or the other, Why?

I have replaced the master cylinder that came with the Right Stuff kit with a "NEW" new master cylinder from NAPA. Right Stuff sent me a replacement booster assy. when the failure was the threaded eye bolt. The bold failed when bleeding the brakes (with one foot) YIKES! Check yours. Once I set the braked peddel hight, I installed nuts on the threads to help prevent a future failure. Adding support to the shaft. The replacement booster came in poor condition. Loose bolts, cotter pins not in safety and the push rod installed backwards......Huh!

Today I will install a 2 lbs. residual valve between the master cylinder and adjustable valve for the rear wheels, bleed, bleed, bleed and let you know.
 

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Boy, Brad, ya got me baffled

You don't have a factory warning lamp / proportioning assy downstream on the frame rail, do you?
 
Boy, Brad, ya got me baffled

You don't have a factory warning lamp / proportioning assy downstream on the frame rail, do you?

Nope. Just the adjustable. Yesterday I installed a 2 lbs. residual valve between the mater cylinder and adjustable valve in the rear brake line. No change. Today I'm going to by-pass the adjustable valve and see what happens.

:banghead::banghead::banghead::banghead::banghead::banghead::banghead::banghead::banghead::banghead::banghead::banghead::banghead::banghead::banghead::banghead::banghead::banghead::banghead:
 
Yesterday I by-passed the adjustable valve and there is a big improvement but, I don't feel it is right. The car is on stands. The rear brakes are more effective but, will not lock up. They slow down, than stop. It is easy to move the rear wheel with a bit of gas peddle.

Might this be a factor:

I have 3/16" brake line from the master cylinder to the coupler (adjustable valve now by-passed) then 6" of AN 4 ( 1/4") line to a firewall bulk head fitting. From the fire wall to the "T" on the differential I have AN 3 (3/16"). From the "T" to each rear disck brake caliper is AN 4 (1/4"). Maybe it's a volume issue & not a pressure issue?????

My understanding is that hydrolic pressure is equil in all directions. I wonter if going 3/16" to 1/4" to 3/16" and finaly 1/4" is having some sort of restriction/expansion effect?
 
If the system is bled AND THE LINES are not expanding (IE thinwall aluminum tube or hoses) Once they are "full" volume in the lines should not be much of an issue. I don't know of any common passenger cars that use larger than 3/16, though.

I'm about out of suggestions, without "being there."

If the system is bled and you have a good pedal, it seems to me this just about has to be a master cylinder issue.

It IS possible to have an air problem or a volume problem whereby you are bottoming the front piston and it would SEEM like a low pressure problem, when in fact you are essentially running the front section "to the end" of travel

Have you actually rigged a gauge to the system to see what you really have for pressure front/ rear?

About 50 bucks should get you a decent enough gauge and some adapter fittings, tee, etc.
 
If the system is bled AND THE LINES are not expanding (IE thinwall aluminum tube or hoses) Once they are "full" volume in the lines should not be much of an issue. I don't know of any common passenger cars that use larger than 3/16, though.

I'm about out of suggestions, without "being there."

If the system is bled and you have a good pedal, it seems to me this just about has to be a master cylinder issue.

It IS possible to have an air problem or a volume problem whereby you are bottoming the front piston and it would SEEM like a low pressure problem, when in fact you are essentially running the front section "to the end" of travel

Have you actually rigged a gauge to the system to see what you really have for pressure front/ rear?

About 50 bucks should get you a decent enough gauge and some adapter fittings, tee, etc.

WEll I'm on my second new master cylinder.....:banghead:

I like the gauge idea. I'm going to try it. Let ya know
 
I have that master cylinder/booster combo & bent the same rod. I also have brake problems but put it down to not enough vacuum. I am testing with extra reservoirs but I wonder if the whole system is a bit suspect after the bent pushrod.
 
Hey Brad426 I have done the following. Used a smaller bore master cylinder to impart more pressure on the discs. Spent a lot of time "bedding in" the pads to the discs. When the front brakes feel better I started adjusting the proportion valve to apply more to the rear brakes. They are pretty good now & I am sure they will get better. It seems that new discs & pads need "bedding in" or they hardly work at all. Now I can use less pressure on the peddle to stop the car meaning less pressure on that suspect rod. I put a bunch of nuts on it just to be sure though.
 
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