brake insanity

[carlwest]

I think I have covered all the posts relating to my problem but have not found the answer.



Brake pedal refuses to firm up and bleed out.



Have done all the prescribed treatments.



new front and rear brakes, bypassed the Rear antilock brake system



new 1 1/8 wheel cylinders 3" drums, turned front rotors and new pads



first tried the rebuilt MC route, then the new MC still no pedal when running



plug MC ports and NO pedal fade, but when reconnect lines, can't get all the air out. when I think I am close restart engine, and pedal goes to the floor.



rear brakes are adjusted tight, and NO leaks in system.



What am I missing????

 

[DValentine]

Thinking out loud here..... Did you bench bleed the new M/C. Do you have something like the mighty vac? You can suck fluid, bleed air by yourself.

d

 

[carlwest]

Yes the first reman I bled on the pickup. This unit was bypassing when I plugged the ports at the MC. The next new unit, I bench bled carefully two different times just to be sure and got no air. I have a helper to depress pedal slowly and another guy watches the MC and keeps fluid level always full. Have tried bleeding with cap on and with cap off.

I was wondering just what the brake combination valve did, so I tried to do some research on that last night. I took it off the truck and cleaned inspected it, couldn't see anything visually wrong. On some makes I learned that this valve also shuts off fluid to the back brakes in the event of a brake failure. Hence when bleeding you have to open a front caliper bleed screw and pop the pedal to re-center the valve before continuing to bleed the back brakes. However I don't think that is the problem because I can get fluid to the back brakes.

I don't get any air when I bleed the front wheels or the left rear wheel. Only the right rear. Somehow it seems the MC is "injecting air" in the system, however that should be impossible.

Could the combination valve under the MC be bypassing? that too seems impossible.

There were about three guys who have had the same problem on here but the ultimate solution I couldn't find. The "Soft Pedal" thread most closely resembles my problem.

Do the hand bleeders work? I have bled with and without the engine running. When I get the pedal to feel firm with the engine off, as soon as we start the engine, pedal fades again.

Today, we try another new MC. This time making sure we don't depress pedal all the way to the floor.

Any other thoughts?

 

[dpuckett]

Are you getting a soft pedal, or low pedal with normal firmess? If the latter, check your pushrod adjustment. Went through this in my 91, AFTER giving my g/f's shop $350 for parts and labor (which I helped with, some discount, huh?).



If it's soft, bench bleed using both methods- plug the ports, then bleed the old fashioned way with the lines going out the ports to the top of the MC. After you get no movement/ air, install on truck with plugs still in.



Hook up lines. Which method are you using to bleed? I use the clear hose on the bleeder going into the bottle of brake fluid. Crack the bleeder about 1/4 turn, and it's a one man job. As long as the end is on the bleeder securely, and the other end is submerged, there is NO WAY for air to get into the system. GO slow pumping. I pause about 5sec between pumps.



ALso replace your soft hoses. They arent expensive, and can cause all kinds of grief, esp when they get to be teenagers.



Good luck, keep us posted.



Daniel

 

[carlwest]

Ok, I changed the MC again, bench bled using the tubes till no air came out. Then waited 20 minutes and did it again until no bubbles. I am not familiar with the plug method. Left tubes on, installed, bled at the top of the combination valve and the bled at the junction located under MC on the frame. Then spent an hour or more off and on slowly bleeding right rear wheel cylinder. Finally got all the air out, but start engine and pedal slowly goes to the floor. I don't push it all the way to the floor since they say that might damage the MC seal.

I measured the push rod protrusion from the vacuum booster MC mounting plate and it was 1 1/8 inch out. Measured the depth of the MC where the point of contact is, to the mounting plate. With the bolts of the MC loose and someone pushes the pedal, there is about an inch of pedal movement before contact with the MC, but that appears to be only pedal slack. If as the MC directions say to only push MC plunger in 1 inch then we are a little past that much movement already

I could try to lengthen the push rod just to see what happens anyway.

Another thing I should mention is with the engine off and the pedal feeling firm, when I start the engine pedal immediately falls away.

The plunger on the rear of the combination valve does move when the brakes are depressed, and I guess that is normal.

I am using the helper bleeding method with a clear hose. No fluid is retracting on each stroke, and finally we got the air out, but still not good pedal.

If my John Deere's gave me this much grief, I would quit farming.

 

[Grizzly]

First of all, include your signature so we know more about your truck.



My only thought at this point, which is a long shot, is that the Master Cylinder could be pushing brake fluid into the booster tank. I think about this remote possibility because it happened to my wife's 03 Camry. A new booster tank took care of the problem.

 

[dpuckett]

Booster tank? I'm lost. If it is part of a hydroboost system, it doesnt apply here- we have vacuum assist.



DP

 

[carlwest]

I cant for the life of me see why the vacuum booster would be forcing air in. I started the engine with the MC off, and had a helper step on the pedal. I didn't used soap, but I cant see, hear, feel and leaks, and the booster holds vacuum for two or three pedal strokes after engine is shut off.

I disassembled the combination valve last night and found a small nick in one of the O rings on the piston. Before I took it apart, I blew air into the plunger side with compressed air and air came out the rear line, or the front brake line. I realize that was compressed air, but could the valve be sucking in air on each stroke?

We bled the brakes for hours again yesterday. Thought I had all the air out a couple different times. started engine, pedal goes to floor slowly.

wait a few minutes, and bleed right rear, and again a endless amount of air.

I read on a brake website that 10 pounds of pressure should be maintained in the rear brake system in order to counteract the return springs on the drum brakes for more immediate brakes, and to keep slight pressure on the wheel cylinder pistons to prevent "breathing" in air.

I wondered if that could be part of the function of the Rear Antilock Brake system.

Im just thinking out loud, I notice this is a common problem.

I also lengthened the push-rod three turns and lost brakes, so I backed it off 1 1/2 turns. Although the brake pedal is higher I am still getting the air and pedal loss.

1992 W250 LE auto, 4x4 cummins
2003 2500 SE auto 4x4 cummins

 

[Paychk]

Do you still have the RWAL system intact? If so under certain conditions the brake accumulator that is located in the valve won't close completely and you will have a dropping pedal. Install the bypass that someone here posted and see what happens.

RWAL bypass:

https://www.turbodieselregister.com...7070-abs-proportioning-valve.html#post1857272

 

[carlwest]

Yes, Paychk, I had read about that being a problem area, and I too pull very heavy loads. I have had the spongy pedal issue for a long time and even with 3 inch drums found little improvement, so the first thing I did was bypass the RWAL. Unfortunately the truck has been on the Paycheck ever since.



I took apart the pressure metering valve as the deal calls it, or the proportioning/combination valve and put a clear hose on the end with the piston rod after I removed the jack stands boot. Then I put compressed air into the lines and and ohm meter on the sender post. The other end of the hose I put in water and tried to prove my theory that it might be sucking in air on when the piston returns, each time the brake is applied. Didn't notice any movement of water, but when I blew air into the piston rod port with the rod/plunger intact just like it operates, I got plenty of air coming out the rear port, or the front brake side.



Maybe that isn't the problem, but in the process of taking it apart, I lost an important washer that when flying across the shop... ... . So I ordered one today, and if anyone wants to know about the combination valve and how it works all donations will be applied to the purchase of the new one.



I am out of ideas and time. I have some more new wheel cylinders, I might try that again as someone in the 2nd Gen post had my same problem and finally found that was the problem. That wouldn't explain the sudden pedal drop though. Today I also plugged the MC ports at several different pedal positions and no movement at all. So I don't think that is the problem.



Thanks for your help guys.

 

[carlwest]

PS, thanks for the link Paychk, and as for the goofs in the last post its late the the spell checker outsmarted me. For those who like the technical terms, I think the valve in the rear RWAL is also the delay valve. The valve under the MC is serves three functions, Warning of brake failure, closes ports to the front or rear whichever should fail, and provides more, about 70% more pressure to the front than the rear. The plunger in the center of this valve for the front brakes is about half the size of the one for the front hence the proper proportion goes to front and rear. etc.

 

[Grizzly]

Booster tank? I'm lost. If it is part of a hydroboost system, it doesnt apply here- we have vacuum assist.



DP

If we're talking about a 1st gen, I believe they have a vacuum booster tank.

 

[Stamey]

When I have encountered this problem it has been a wheel cylinder that was defective. I had one on my 1974 F350 that the wheel cylinder had come apart on the left side, but I got no air when bleeding on the left side, and no noticable fluid leak. But when bleeding the right side, could never get rid of all the air. I never looked for a wheel cylinder problem because I saw no fluid leaks, and didn't imagine a wheel cylinder could suck air... ...

On my 91 W350, a brand new wheel cylinder from Napa began seeping as soon as I finished bleeding it. I am quite glad I did not put the drums back on before bleeding the system (I was gravity bleeding without the drums on).

My question, regarding the pedal going down when you start the engine, is, do the brakes engage when the pedal goes down by itself, or not?



Chris

 

[carlwest]

Thanks Stamey, I have a couple new wheel cylinders and will check that today. Yes the brakes do engage as the pedal creeps to the floor.

 

[Stamey]

You need to look hard at the brake booster because that's what's pulling the pedal down. I have encountered this before, but I cannot remember the exact circumstances because it was about 20 years ago. It was on a gasser and the vacuum on the brake booster was applying the brakes on the car. It may have been a bad/weak spring in the brake booster, but I just cannot remember exactly. I believe we replaced the brake booster to fix it.



Chris

 

["Mad Max"]

well, my initial thoughts are that there is still air in the system somewhere, and one place AI can think of is the front calipers. In the past I have had to remove the calipers, fronts and rears (if so equipped), and simple invert the calipers and whack on 'em a couple times with a wrench or other non-approved whacking device, which has actually released trapped air bubbles buried deep in the calipers. I have actually had positive results with this procedure, so that may be a possibility.



If you have no leaking fluid anywhere then the system HAS to have air trapped in it somewhere - has to, and there's only so many places where air can get trapped.



- M2

 

[Paychk]

I have seen the calipers installed on the wrong side and the bleeders were pointing down, the guy had spent a week trying to bleed those out. #@$%!

 

[carlwest]

Oo. I finally have brakes! Not sure exactly what the problem was, but replaced the combination, proportioning valve, and put two more new wheel cylinders on the rear, sense it is so much fun, and lastly used a Lincoln suction gun, to pull a strong vacuum on the bleeder ports. Did that about five times each wheel for good measure... and Wow! Really nice pedal! Will have to wait for the water to subside around here to try it out. The Lincoln hose fit snugly over the clear vinyl stub hose I had slipped over the bleeder port.



Thanks for all who offered help, I hope this will help others with the same problem. Don't forget to leave the MC lid off when you bleed. I had the parts pro tell me he left it on, however the amount of brake fluid it takes to charge the system the vacuum produced on the MC will cause air intake on the MC plunger rod and the plunger rod on the rear of the combination valve. At least that is my theory.

 

[puff1]

I had my truck down for 3 months because of the same problems. Went through a gallon and a half of brake fluid 3 boosters and 3 masters. I kept using cardone and it always turned out to be the stupid new parts. The third master I had which was brand new turned out to have a rear leak which in turn destroyed the booster diaphram. I was about to give up but then decided to switch brands it seams to have worked. I also resealed my vacume pump which should be running at about 28 inches Hg. A quick test for your check valve is letting you truck run for a while, let the vacume build up then shut the truck off without touching the pedal. take your check valve and give it some side pressure if the booster is working you should hear some air rushing in. Pull your master away from the booster without removing the lines and check for a leak you will see fluid near the front booster seal. Also check all your vacume lines. Don't use cardone!!! Do you know about the metering valve? I believe its between 3 and 117 psi that it shuts off the front half of your brake system, if power bleeding you need a special tool to hold it open or in essence you will only be bleeding half the system. This means you need to either farmer bleed or go really slowly and not pressure up the system when using the jar and tube method. Maybe this will help? or really mess with your head

 

[Wickstead]

Good luck with your fixes, I'll be interested in your results and will watch this post so please keep us updated on the progress.



For reference, found a link on how the rear ABS unit works... for those of you that have not bypassed the system or just curious.



Kelsey-Hayes RWAL Antilock Brakes



Kelsey-Hayes RWAL Antilock Brakes



Sending this from the RV park with Wi-Fi and my brake pedal going to the floor. Thank goodness the fiver has dual electric brake axles (not recommended - I will be fixing the brakes ASAP with the info from the above link)!



PS - the napa part for the

"Anti-Lock Brake System (ABS) Modulator Valve - Rem" is UP 570044.