Brakes, vacuum, RABS, master cylinder...

[bucholzi]

I'm messing around with my brakes since I have sinking pedal, but I cannot pinpoint the problem.. I got plenty of brakes, but when the motor is running and I pump the pedal and holds firm pressure, the pedal sinks slowly. It does not sink all the way to the floor, but almost, and I never loose brake pressure. When the engine is off I get solid pedal with absolutely no sinking.

I have bled out the entire system (including the RABS). I have new braided hoses, new pads, new rotors, new drum cylinder and new shoes. Rear brakes adjusted to slight drag.

I have tried to pull the spring from the RABS, still the problem persist. This should indicate that the RABS is not the problem

When I run the engine and the pedal sinks, the level in the master cylinder reservoir remains the same: does this indicate bad MC I don't have a plug that fits the master cylinder, so I haven't tried to plug the ports to check for intern leakage.

But I got a vacuum gauge in the truck, and if I pump 2-3 times the vacuum drops low (down to 5 psi) and as the vacuum builds up again to 20-25 psi the, the brake pedal sinks. And it seem to stop as the vacuum stabilizes. Can it be the brake booster working harder as the vacuum increases

I seem to recall somewhere that this sinking pedal is normal behavior for these trucks, but that ford came with a alternative master cylinder (not as a recall, but to make the complainers happy) that removed the symptom? Any truth in this, or is my memory playing tricks with me?

 

[LCAM-01XA]

I remember hearing the same about the redesigned parts, the redesigned master cylinder has a larger bore diameter IIRC.

The way I see sinking pedal, it's always result of a leak - fluid don't compress, if pedal goes down the fluid has to be going somewhere - whether it's an internal RABS HCU leak, internal master cylinder leak, or an external brake line or hose or caliper or wheel cylinder leak, it's still a leak. Now about finding it, well the external ones are easy to spot, and you already tested the RABS HCU by locking the accumulator chamber piston at the bottom, so the only thing left is master cylinder. I've never been able to notice any fluid level drop in a master cylinder when brakes are applied cause I always get the fountain shot out of the bypass ports, so now I don't even try no more - sinking pedal in my experience almost always is caused by a faulty master cylinder. As for the engine's effect on it - while running the booster puts a whole lot more force on the master cylinder pistons than you can on your own, so maybe the seals hold up fine to your pushrod force but throw the booster in the mix and suddenly the pressure is too high for them to keep contained and the internal bypass happens and your pedal sinks.

 

[m67tang]

I had the same trouble about a month ago. I replaced the MC ,,which had a life time warranty from raybestos ,and it fixed the problem

 

[oldmisterbill]

I feel that many mastercylinders have been replaced when the real problem is The RABs valve.It gives the same sympton as a bad master cylinder.When I bypassed my RABS it took care of the slow sinking pedal. & yes I want to get Rabs vaklve back on it.I liked it better with the antilock.In my 1984 F250 I had no RABS it was not a problem,but for whatever reason thebrakes are very touchy on my 1991 F350 without RABS. IMHO Mr Bill

 

[LCAM-01XA]

Bill, it's my understanding that your '91 brakes are so touchy cause the prop valve on the master cylinder don't reduce the rear line pressure as much as the one in your '84 - basically with the '84 Ford played it safe and reduced the pressure a lot to make sure you won't be locking up the brakes easy, while with the '91 they gave it more juice and let the RABS dial it down as needed.

And he said he already pulled the spring off the RABS HCU, what that does is prevent the piston in the accumulator from returning the fluid back in the main circuit - basically the first few pumps on the pedal it will be soft, but then the accumulator fills up and that's it, no more fluid goes in it and no fluid leaves cause the piston has bottomed out, thus effectively disabling the RABS system and achieving the same thing you did when you actually bypassed the HCU.

 

[franklin2]

On my truck that had the RABS, I don't have a proportioning valve.

When I bypassed my RABS, I had to run the rear line straight to the master cylinder, there was no proportioning valve in the system. I know all trucks are a little bit different, but mine stops fine like this, one of the rear wheels would lock up under panic braking even with the RABS hooked up anyway.

 

[bucholzi]

I remember hearing the same about the redesigned parts, the redesigned master cylinder has a larger bore diameter IIRC.

The way I see sinking pedal, it's always result of a leak - fluid don't compress, if pedal goes down the fluid has to be going somewhere - whether it's an internal RABS HCU leak, internal master cylinder leak, or an external brake line or hose or caliper or wheel cylinder leak, it's still a leak. Now about finding it, well the external ones are easy to spot, and you already tested the RABS HCU by locking the accumulator chamber piston at the bottom, so the only thing left is master cylinder. I've never been able to notice any fluid level drop in a master cylinder when brakes are applied cause I always get the fountain shot out of the bypass ports, so now I don't even try no more - sinking pedal in my experience almost always is caused by a faulty master cylinder. As for the engine's effect on it - while running the booster puts a whole lot more force on the master cylinder pistons than you can on your own, so maybe the seals hold up fine to your pushrod force but throw the booster in the mix and suddenly the pressure is too high for them to keep contained and the internal bypass happens and your pedal sinks.

No external leaks, rabs was also dry inside. The truck has always acted like this, but while preparing for the truck getting inspected on friday (the 13th...) everything is getting a closer look at, especially since the truck has been sitting for more than 4 years

I will try to bleed the brakes once more, and order up a new master cylinder regardless if the brakes passes inspection or not.

 

[LCAM-01XA]

On my truck that had the RABS, I don't have a proportioning valve.

When I bypassed my RABS, I had to run the rear line straight to the master cylinder, there was no proportioning valve in the system. I know all trucks are a little bit different, but mine stops fine like this, one of the rear wheels would lock up under panic braking even with the RABS hooked up anyway.

Right, and when you ran the line to the master cylinder, was there one helluva big and fat fitting in there already that you hooked the line to? That's the prop valve, it's not the older style that ties the front and rear circuits together and has the bias switch inside it that triggers the red light on your dash when one of the circuits dumps the pressure. I didn't know that either, and for the longest time I though with RABS there ain't no prop valve, but there is - that valve is actually of the same style used on many late-'90s fullsize passenger cars, even those with no ABS at all.

Also since the RABS takes readings off the tone ring on the diff, it has no way of knowing if one of your wheels is locked up, it will only dump the pressure if both wheels lock as that's when the diff stops turning too. Maybe one of your brakes is adjusted a bit tighter than the other?

 

[icanfixall]

This is a classic problem with the rear self adjusters not doing there job. Your truck is now stopping with just the front brakes and that will overheat them. What happens is just like you described. You apply brakes... Come to a safe stop but... While sitting stopped the peddle slowly sinks down. Whats happening is the rear shoes are so far out of adjustment they are not filling the wheel cylinders during braking. As you sit there they slowly fill and that drops the brake peddle. Want to check this.... Apply the parking brake. Dose it set very near the floor... If so then the shoes are way out of adjustment. Use a brake spoon and manually adjust the rears. Test the parking brake again or... Apply the parking brake just enough so you can still drive the truck. Do this so you can safely brake. Now drive around slowly and apply the brakes and come to a stop.... Did the peddle sink down to the floor... Now you can se whats going on with the poorly designed self adjusting rear brakes. A larger master cylinder will help with tis because it can push a larger volume of fluid to the brakes but... Its not going to be a higher pressure of fluid. Thats not an issue. The vacuum brakes apply around 1000 lbs of fluid pressure to the brakes. The hydroboost brakes apply 2000 lbs of pressure to the brakes. Several members have upgraded to the hydroboost brakes with the 1 5/16 master cylinders... They stop very well in a controlable way....

 

[bucholzi]

This is a classic problem with the rear self adjusters not doing there job. Your truck is now stopping with just the front brakes and that will overheat them. What happens is just like you described. You apply brakes... Come to a safe stop but... While sitting stopped the peddle slowly sinks down. Whats happening is the rear shoes are so far out of adjustment they are not filling the wheel cylinders during braking. As you sit there they slowly fill and that drops the brake peddle. Want to check this.... Apply the parking brake. Dose it set very near the floor... If so then the shoes are way out of adjustment. Use a brake spoon and manually adjust the rears. Test the parking brake again or... Apply the parking brake just enough so you can still drive the truck. Do this so you can safely brake. Now drive around slowly and apply the brakes and come to a stop.... Did the peddle sink down to the floor... Now you can se whats going on with the poorly designed self adjusting rear brakes. A larger master cylinder will help with tis because it can push a larger volume of fluid to the brakes but... Its not going to be a higher pressure of fluid. Thats not an issue. The vacuum brakes apply around 1000 lbs of fluid pressure to the brakes. The hydroboost brakes apply 2000 lbs of pressure to the brakes. Several members have upgraded to the hydroboost brakes with the 1 5/16 master cylinders... They stop very well in a controlable way....

The rear brakes are adjusted to barely scuffing (and the parking brake is adjusted per shop manual instructions and works well) and I can easily get the rear tires to lock on gravel if I slam the brakes. So I don't think the rear is the problem. For now, at least: I can't stand the self-adjuster crap which I think only works on one side on this truck. I have never gotten the rears to work properly on any of the trucks I have had. Usually one wheel starts to lock up and then it's back under the truck to adjust again

 

[oldmisterbill]

Bill, it's my understanding that your '91 brakes are so touchy cause the prop valve on the master cylinder don't reduce the rear line pressure as much as the one in your '84 - basically with the '84 Ford played it safe and reduced the pressure a lot to make sure you won't be locking up the brakes easy, while with the '91 they gave it more juice and let the RABS dial it down as needed.

And he said he already pulled the spring off the RABS HCU, what that does is prevent the piston in the accumulator from returning the fluid back in the main circuit - basically the first few pumps on the pedal it will be soft, but then the accumulator fills up and that's it, no more fluid goes in it and no fluid leaves cause the piston has bottomed out, thus effectively disabling the RABS system and achieving the same thing you did when you actually bypassed the HCU.

In my situation I had lost all my brakes. I replaced the MC bled & bled. Next replaced MC still had brak problems ,but finally got brakes. A few thousand miles later I lost tthem again (while on the road) they replaced the MC again,after much bleeding got brakes . A couple thousand later in St Louis -no brakes bled & bled got brakes .They lasted 200 miles. Drove home depending on trailer brakes, Thought situation over -unplugged rabs -bled still no brakes -bypassed RAbs and got hella brakes-no problems since. I do have the slowly sinking pedal when you mash then while sitting still. When on the road you will never notice the sinking pedal as it has plenty of pressure to do its job. I'm not sure (guess I need to investigate more ) but i think the RABS can fail internally allowing it to bypass when it shouldn't.
I wonder for knoledge sake. How many of you experience a slowly (very slowly) sinking pedal when stopped just mashing on the pedal and holding it? Yet find it stops fine on the road?

 

[LCAM-01XA]

In my situation I had lost all my brakes. I replaced the MC bled & bled. Next replaced MC still had brak problems ,but finally got brakes. A few thousand miles later I lost tthem again (while on the road) they replaced the MC again,after much bleeding got brakes . A couple thousand later in St Louis -no brakes bled & bled got brakes .They lasted 200 miles. Drove home depending on trailer brakes, Thought situation over -unplugged rabs -bled still no brakes -bypassed RAbs and got hella brakes-no problems since. I do have the slowly sinking pedal when you mash then while sitting still. When on the road you will never notice the sinking pedal as it has plenty of pressure to do its job. I'm not sure (guess I need to investigate more ) but i think the RABS can fail internally allowing it to bypass when i souldn't.

Bill, you are 100% correct, the RABS can sometimes bypass internally - what happens is one of the valves leaks past its seals and when you stand on the brakes fluid bleeds out of the main circuit and into the accumulator chamber, where all the line pressure you're trying to build up works against a single small spring - think of it as another rear brake but without a drum on it, every time you push on the pedal it soaks up your line pressure, but every time you let off the spring pushes the fluid right back in the main circuit. Now when you pull the spring the fluid still bypasses the valve and goes into the accumulator chamber, but when you release the pedal it don't get pumped back in the main circuit and the master cylinder compensates for the "lost" volume from its own tank - after a few pumps the piston in the accumulator chamber has bottomed out and there's physically no more space for fluid to enter in there no matter how hard you stand on the pedal, so the RABS now starts acting like a rear brake with a seized wheel cylinder and all the pressure you build up with the master is sent right down the line like it should.

When I test a RABS for that internal leak I actually don't simply pull the spring, instead I replace it with a solid spacer, this way the piston is locked in place from the beginning and fluid even if it tries to leak past the internal valve simply has nowhere to go, thus no pressure drop in the system.

I wonder for knoledge sake. How many of you experience a slowly (very slowly) sinking pedal when stopped just mashing on the pedal and holding it? Yet find it stops fine on the road?

I had that, replaced the return spring for the piston in the RABS with a stiffer one, and issue went away - not sure how or why, but I never experienced it again, even after the hydroboost install

 

[oldmisterbill]

Bill, you are 100% correct, the RABS can sometimes bypass internally - what happens is one of the valves leaks past its seals and when you stand on the brakes fluid bleeds out of the main circuit and into the accumulator chamber, where all the line pressure you're trying to build up works against a single small spring - think of it as another rear brake but without a drum on it, every time you push on the pedal it soaks up your line pressure, but every time you let off the spring pushes the fluid right back in the main circuit. Now when you pull the spring the fluid still bypasses the valve and goes into the accumulator chamber, but when you release the pedal it don't get pumped back in the main circuit and the master cylinder compensates for the "lost" volume from its own tank - after a few pumps the piston in the accumulator chamber has bottomed out and there's physically no more space for fluid to enter in there no matter how hard you stand on the pedal, so the RABS now starts acting like a rear brake with a seized wheel cylinder and all the pressure you build up with the master is sent right down the line like it should.

When I test a RABS for that internal leak I actually don't simply pull the spring, instead I replace it with a solid spacer, this way the piston is locked in place from the beginning and fluid even if it tries to leak past the internal valve simply has nowhere to go, thus no pressure drop in the system.


I had that, replaced the return spring for the piston in the RABS with a stiffer one, and issue went away - not sure how or why, but I never experienced it again, even after the hydroboost install

Now if I had had all that information 6 years ago I wouldn't have cut and revamped my lines to by pass the rabs valve. Great information!