ZAAR

Had the same problem recently seemed like I wasn't getting vacuum assist on the booster? so I tested the vacuum from the diaphragm. I had the same hard pedal and all so I replaced the booster and master... whadda ya know wham it worked. Remember if you try to stop a vehicle without vacuum assist you better have strong legs and can kick like a mule lol. but that's my experience please go through everyones comments here cause my experience may not be the same as yours.
good luck.

BCRick

I've been gone and busy for the last couple of weeks and drove the truck for the first time today and the issue has not healed itself. I have to ask Marf75 about the full lock to the left issue. It is real! I tried it a few times today and it really does happen when backing or going forward with the wheel at full lock to the left. You didn't go into any detail but do you have any other explanation why it happens? The truck is going back to the shop next week for rebleed, rebleed and see if that is the problem but if there is some relationship to the wheel being hard to the left I would love to hear it.

MARF75

I don't have any more info or remedies for this issue.

I would love it if someone came up with something, but in my travels across the internet , I have came up empty. I'm going to swap to hydroboost as an attempt to fix the problem, when the funds are available of course.
Sorry I can't help

1999 duallie

Has anyone mentioned "Bench Bleeding" the Master Cylinder?

MARF75

Yup!
Tried it to no avail

MARF75

I find it hard to believe its related to air in the system.
It's perfectly normal brakes and then out of the blue, a hard left turn, andthe pedal goes to the floor with very,very little braking force and then if you let the pedal all the way back up you usually have good brakes again.

1999 duallie

My 1.030" ID. MC in my racecar took 40 full pumps circulating back into the reservoir under the level of the fluid for a return syphon effect, to bench bleed to full fluid flow. Did you go this far?

Another issue can be bleeding in the wrong order. Must bleed from RR - LR - RF - LF. ie furthest from MC to closest to MC.

ZAAR

hey are you sure that booster is in good shape?
are you getting vacuum?
and are the lines to the front calipers good?
soft lines do collapse over time and age.
I think the process is to go over the whole system before you start trying to figure out which one item would be defective. and normally if one caliper is defective you should also change the other too.
and lines.
proper bleeding of the master and lines will help the issue.
I am sure the stuff I am saying you've already went through, but try overlooking the whole system and not spend your time on one item. The brake system is not very complex you don't have too many extra items IE abs etc.
keep it simple and don't overthink just start from simple and move around sometimes it takes walking away and trying something else.

brainfade

If the issue is repeatable (ie. happens every hard left or only on a hard left) the I'd be looking at the caliper slides and wheel bearing adjustment. A loose wheel bearing in combination with a caliper frozen in the slides and a hard turn can cause the rotor to push the piston back into the caliper and it will take a few pumps to get it out again.

BCRick

Truck was back into the shop this morning, proper bleed sequence was performed, brake mechanic with over 30 years experience and owns and drives a 92 W250 also found the wheel bearing was one turn loose. He also thought the play could compress the pad and be possibly causing the pad to stick. Left caliper was replaced the first time in the shop. Pedal feels strong with 2/3 pedal. He took it for a ride and says feels good. He gets in the truck with me and I drive around the block with good brakes into a parking lot, slow turn hard over left on the steering and the pedal goes to the floor. Did it in reverse, same thing. 3 slow pumps to get about 1/2 pedal and a little soft.
Drive straight ahead or right turns, no problem. He and the shop foreman have no clue, never been seen. Can't think of a way that steering can effect the brakes like this. Lines look good, no kinking or rubbing. I am going to get the fronts at least done soon about 3mm left but we would all like to figure this out instead of just replacing things. Always looking for any other experience like this besides MARF75.

j_martin

The disk brakes normally back off only about .005". It's just the difference between a slightly upset square rubber seal, and settling in square.

Any runout in a disk, either caused by a warped disk, loose disk mounting, or wheel bearing looseness can push the piston back into the caliper considerably more than .005", and it can take several strokes on the master cylinder to get it back.

Another cause of random low pedal is the rwal accumulator valve leaking, but that usually is only 1/2 to 1 stroke on the pedal.

maybe368

I have pretty much stayed out of brake threads because I was accused of saying that a disk brake conversion is the only way to solve the brake problems on these trucks. I never said that, but it did fix my problems. I had very similar problems to yours, It would suddenly go to the floor, mostly at slow speed in reverse, but it did happen a few times while driving. I threw a lot of parts at it, bled them until I bled, did everything you did and more. My brakes have never worked better since I converted to disks a few years ago. If I had spent the money that I spent on all those parts on the conversion , I would have been ahead. The Only thing that I eliminated was the drum system, I still have the rear brake anti-lock valve and the speed sensor. So, I hope this helps...Mark (disclaimer: I am in NO way saying that disks are the Only way to fix this problem. it just worked for me) Peace out

j_martin

RWAL is the problem for a mystery (unexplainable) low or no pedal problem 80% of the time.

Here's the test to confirm or eliminate the RWAL as the problem.

1. Remove the plug in the end of the accumulator in the RWAL valve.

2. Insert a long wire and check for movement of the piston when the brake pedal is depressed.

3. If the piston moves, the RWAL valve is leaking.