brakes falling pedal
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Senior User
Joined: Mar 2007
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From: At Work!
brakes falling pedal
hello all Have a good question for you?
1992 f-150 straight 6 manual tranny on second engine and fifth master cylender!!!!!! from oreilly auto.....
A....does anyone have any special things they do when replacing master cyl..
B...When you press brake pedal(engine running)it slowly falls
C...Would the booster be at fault or is this possibly the fifth bad master cylendar from orielly's that i have recieved?????
I am bleeding from furthest to closest ect... any suggestions!!!!!!!!!!!!!!!!!!!!!?????????????????? ?????????????????
1992 f-150 straight 6 manual tranny on second engine and fifth master cylender!!!!!! from oreilly auto.....
A....does anyone have any special things they do when replacing master cyl..
B...When you press brake pedal(engine running)it slowly falls
C...Would the booster be at fault or is this possibly the fifth bad master cylendar from orielly's that i have recieved?????
I am bleeding from furthest to closest ect... any suggestions!!!!!!!!!!!!!!!!!!!!!?????????????????? ?????????????????
Lead Driver
Joined: Aug 2006
Posts: 5,670
Likes: 13
From: Sand Lake, MI
Do you "bench bleed" the master cylinder before connecting and then bleeding the lines?
The term "Bench bleeding" is kinda misleading if you're not aware, it can be done on or off the truck. Solid plug the line outlets and cycle the piston until you see no more bubbles or use a kit. I prefer the kit, very cheap to buy and works without fail and can see the results as you use it better.
If you've done that, bench bleed it, then it sounds like you need a new MS again. Maybe get one from another source?
The term "Bench bleeding" is kinda misleading if you're not aware, it can be done on or off the truck. Solid plug the line outlets and cycle the piston until you see no more bubbles or use a kit. I prefer the kit, very cheap to buy and works without fail and can see the results as you use it better.
If you've done that, bench bleed it, then it sounds like you need a new MS again. Maybe get one from another source?
Post Fiend
Joined: Jan 2005
Posts: 8,402
Likes: 10
Spot on. I had the same problem as you describe, a good brake pedal untill stopped at the lights or wherever and the pedal would slowly sink to the floor. It turned out to be a faulty solenoid valve in the RABS valve body allowing fluid to flow into the chamber with the spring loaded piston. If you think that may be your problem there is an easy test. On the front of the RABS valve body there is a large hex plug. Unscrew that plug (be careful it is under spring pressure all the way) and remove the spring. Replace the plug. Pump the brakes and if necessary bleed the RABS valve. If pedal no longer sinks then the valve body is faulty. I drove mine for two years before I replaced the RABS valve body.
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Senior User
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From: At Work!
Spot on. I had the same problem as you describe, a good brake pedal untill stopped at the lights or wherever and the pedal would slowly sink to the floor. It turned out to be a faulty solenoid valve in the RABS valve body allowing fluid to flow into the chamber with the spring loaded piston. If you think that may be your problem there is an easy test. On the front of the RABS valve body there is a large hex plug. Unscrew that plug (be careful it is under spring pressure all the way) and remove the spring. Replace the plug. Pump the brakes and if necessary bleed the RABS valve. If pedal no longer sinks then the valve body is faulty. I drove mine for two years before I replaced the RABS valve body.
is this the proportioning valve????
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Senior User
Joined: Mar 2007
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From: At Work!
I removed spring and no more sinking pedal ,but!!!! my pedal falls half way down before pressure is felt...I don't know but I think we are going in the right direction.I have to wait till tommarow to get help to bleed system then will reply but I wan t to klnow exact name of valve
New User
Joined: Aug 2011
Posts: 12
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From: South Central PA
Need help with this same issue in a 1992 F-350 dually diesel. We have replaced the master cylnider, the booster, and the RAB valve, calipers and wheel cylinders do not leak and the pedal still drifts. We have great brakes, but once you are stopped and holding your foot on the brakes they begin to drift to the floor. Any ideas what it could be?
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