Odd Brake Issue: pedal? system?..... input?

[Coley]

Ok...I have a very odd brake issue that I first started to notice last year.

First, some basic correct information:
-The truck is '72, 2wd, 1/2 ton with power brakes (discs on front)...all original.
-I have NO loss of brake fluid in the system...reservoir(s) are full.
-The brake pads and shoes are in good condition (+50% or more for sure)
Now:
-When I push the brake pedal down it hits a stopping point (midway?)....for about 5-10 seconds...then if pressure is continued (not lifted) it heads for the floor and the truck starts to creep.
Now, if I lift my foot and reapply pressure I get the pedal and the brakes back.

So...if the system was using fluid (losing it)...I might think I had air in the system, but there is no loss of fluid.

As well, the brakes are extremely 'grabby' when the truck is first started and moved...then they settle down within a minute or two.
After that I drive the truck all over the place, but I am aware that when I am at a light, I have to re-apply the brake pedal at least once to keep from creeping after initially stopping.
If I 'jam' the brakes while driving, etc.....they definitely grab.
so.....
Does this sound like the proportioning valve is working properly?
Is there a good way to analyze and check this?...a few intelligent, proven tests or?

Many Thanks!
Coley

[weq92f]

Most likely master cylinder.

-klb

[Coley]

...any chance of being more specific?

I am trying to solve what exactly is going on here....rather than just starting to replace parts until the issue goes away.....noting that is often the way most 'repair centers' (so called) go about fixing things, lol.

Many Thanks
Coley

[Steeveedee]

That is a classic symptom of a bypassing master cylinder. The key is that no fluid is lost. Replace it! I wish they were all this easy to diagnose.

[Coley]

Originally Posted by Steeveedee

That is a classic symptom of a bypassing master cylinder. The key is that no fluid is lost. Replace it! I wish they were all this easy to diagnose.

Interesting....so what is going on within the 2 chambers that is bypassing?
...is the bypass within the master cylinder?....or is it the proportioning valve?

again, interesting stuff...I'm trying to imagine what the cross section of the master cylinder looks like and what is happening here....hmm

Many thanks!
C

[Bigdav160]

Usually you can see the levels in the MC start rising in one chamber with a bypassing MC.
Have some one hold down the brake pedal and look in the master cylinder reservoirs.

But yes. I have seen one proportioning valve act like your symptoms. The "grabby"ness is a possible sign.

[weq92f]

imagine a syringe full of fluid. You push the plunger to pressurize the fluid. And hold it. Now imagine the fluid seeping past the plunger and allowing the pressure to bleed off.

This is what happens inside the master when your foot goes toward the floor under constant pressure.

I am not familiar with the prop valve fail condition though.

Hth

-klb
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[Jrainman]

I had this same excact issue , and yes you pray everyday that you hope you dont need to stomp on the brakes .because you are not sure you will stop in time . So I replaced my master thinking it was a bad master it stiffin up the feel a bit but ,I could still push the pedal to the floor . then one day 6 months later one of my calipers locked up ,soI changed the caliper and then I could push the pedal through the halfway mark but it did not go to the floor ,the next day I changed out the the other side caliper and that made everything 100% Nice high pedal no to the floor firm and nice responsive brake system....

[Coley]

Well so far....looks like it could be a variety of things.
Now, as far as braking goes....if I stomp on it...it stops, it will lock up actually if i hit it hard enough.
If it didn't stop or was vague....i wouldn't drive it.
It is after it stops....that the pedal starts to float down....

Coley

[B. W.]

Originally Posted by Steeveedee

That is a classic symptom of a bypassing master cylinder. The key is that no fluid is lost. Replace it! I wish they were all this easy to diagnose.

Spot on, master cylinder is bypassing internally.

replace the master.

[Steeveedee]

Originally Posted by B. W.

Spot on, master cylinder is bypassing internally.

replace the master.

Yes, and eventually you will hit the pedal and nothing will happen, in terms of slowing down.

[Myoldtruck]

How old is everything else in the system. I agree about changing out the master cylinder. Keep in mind that its very reasonable to replace all the components of the braking system and keep them up to date. New brake hoses, wheel cylinders, calibers, quality brake shoes and pads. Clean everything really good inside the drums.

[El Dorado Jim]

Originally Posted by Steeveedee

That is a classic symptom of a bypassing master cylinder. The key is that no fluid is lost. Replace it! I wish they were all this easy to diagnose.

x2, change the master cylinder immediately! easy to change, bench bleed the master cylinder,change it and you don't have to bleed the lines....

[71CHEVYSHORTBED402]

Originally Posted by El Dorado Jim

x2, change the master cylinder immediately! easy to change, bench bleed the master cylinder,change it and you don't have to bleed the lines....

Didn't know that. Great idea. Not to hijack the thread, but I have to ask........On completely new brake system, you have to bleed the lines no matter what, right?

[El Dorado Jim]

Originally Posted by 71CHEVYSHORTBED402

Didn't know that. Great idea. Not to hijack the thread, but I have to ask........On completely new brake system, you have to bleed the lines no matter what, right?

Yes you have to bleed all the lines on a new system, starting with the closest wheel moving to the next and doing the farthest wheel last, when you get a new/ rebuilt master cylinder you should get a couple of plastic fittings that screw into where your brake lines go, sometimes they are just plastic plugs and sometimes they are fittings to with plastic tubes that you run back into the reservoir on the MC, put the MC in a vice, not too tight or you can ruin it, fill the reservoir with brake fluid and with a large Phillips screw driver or rod with rounded end, slowly push plunger in all the way, let it back out slowly also, repeat this until you don't see any bubbles coming up through the fluid in the reservoir, now the MC is bled and you can install it on the truck
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[Coley]

So if is the master cylinder...is the fluid that is being compressed by the plunger simply 'cycling' over into the secondary brake system and back? ...noting that the fluid must be going somewhere and it isn't compressible.
Any ideas or description as to what is going on within that master cylinder would be great...

many thanks!
Coley

[weq92f]

The hydraulic fluid doesn't compress unless it has air in it. The "plunger" is not sealed in the bore which is allowing the fluid to pass by the plunger.

Refer to the syringe explanation earlier in this thread.

When you plunger completely fails ( it is partially failed right now ), pushing the brake pedal will not cause anything to happen ( no brake at all ).

Hth,

-klb