BCRick

I had a pedal going all the way to the floor. Local shop replaced the master cylinder and one caliper which had a broken bleeder fitting. The brakes need new front pads and these jobs often find other things and funds are a little short so waiting for a couple of months.

I had half a pedal after the work but the brakes felt okay. Then found that in the course of my day going to town, the pedal would occasionally (2 or 3 times) during the course of the day go to the floor with minimal (almost none) braking. It would take 2 or 3 slow pumps to get anything. After this the brakes would be good until it happened again. Needless to say I left lots of room when coming to a stop.

The shop was at a loss to explain what might be the cause as it was so intermittent they drove it and couldn't get it to happen. They took it back yesterday and adjusted everything up tight. The rest of the system checked out as far as they could tell. They didn't want to just start replacing parts until they could get a diagnosis. The brakes worked fine on the way home with no sign of fade and a full strong pedal.

Anyone have any ideas what could cause a symptom like this?

bobva

Rebleed rebleed rebleed rebleed.
Bleed the RWAL dump valve Also

I just went though the same as you.

rebleed rebleed rebleed.

BCRick

Thanks for that Bob, if it happens again I'll know what to tell them. Still hard to understand how it can be so random. I would think it would have some pattern to the failure.

j_martin

It's a combination of 2 items.
1. Clearance or air in the lines .... needs 3 strokes to compress.
2. Master cylinder adjustment... the piston isn't coming back far enough to clear the feed port and so holds the fluid in the airy or loose system.

Occasionally, it'll bleed down, seemingly at random.

To find the fault, when the pedal seems normal, crack the lines at the MC. The one that spits at you is the defective circuit.

Almost gave my life to learn about the MC adjustment thing with a loaded 1950 Chevy dump truck.

MARF75

Good luck!
See if it coincides with full lock,left turns.
That's all I'll say.

BCRick

Thanks all, once again this group is the best source of info anyone could ask for. The explanation of the randomness makes sense and the shop manager will see a coppy of these replys.

The comment about full lock turns is interesting. I did have it happen a couple of times when I had the wheel hard over in reverse and then had the pedal failure.Thanks for the hint, I will definately do some more experimenting.

Jim Lane

It could still be the master cylinder is bad or the pushrod out of adjustment, I had installed and and bled 3 remanufactured Napa master cylinders all bad from A1 Cardone until I bought a new one for my Napa brake booster combo.

Is your brake pedal tight against the dash or is it weak, it might not be allowing the master cylinder to return to rest position.

MARF75

Please report any findings. Id be interested to know. I'm stumped with a very similar issue.

TJE

I've had similar issues never fixed UNTIL I got rid of the RWAL valve! Problem solved.

MARF75

No RAWL valve here. Anyone??

BCRick

I did insist on a new, not remanufactured master cylinder. Read too many threads on remans not being reliable.

bobva

This is what i did on my final bleed.

the whole system was gravity bleed. RWAL Rears & fronts. Truck would stop on a dime everytime, Then the peddle would sink to the floor evertime.

I removed lines from master cylinder caped it with brass plugs.

Then I tightened the lines on the the proportioning valve filled lines with brake fluid via the use of a syringe and taping on the line to get the air out.

Let it sit over night. (air rises)

In the morning reconnected lines to MC gravity drained lines to PV tightened when no more air was seen coming out.

Now good firm peddle.

bobva

Jim

Pushrod out adjustment? A little more detail please.

Smokin$$away

bobva I think what Jim is talking about is the push rod that's in front of the brake booster that pushes the master cylinder. I've had this issue on other trucks before. I'm not sure where I read the correct adjustment procedure, but it wasn't too difficult. If you have too much gap in the adj rod your brakes will be way down, if it's out too far your brakes can start dragging from not releasing. Pita to figure out, but after burning up pads calipers and rotors and bearings I figured it out!!!

TJE

If ur RWAL valve is working.....that is one thing. If it is toast no amount of bleeding is gonna give a firm pedal. IMHO
Good luck