Brake problem
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Senior User
Joined: Feb 2006
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Brake problem
I had a leaky master cylinder on my 91 F 250 diesel so I changed out the master cylinder, bled the brakes and the brakes were locked up. I opened the R rear bleeder and the brakes released. Tried them again and they locked. Opened the line off the master cylinder to the rear brakes and it released them. So, I figured I bought a bad master cylinder. I exchanged it today and the same thing.
When I turn the key on the rear antilock light comes on and when it goes off I hear a "bump" noise at the same time.
HELP!!
Thanks in advance.
When I turn the key on the rear antilock light comes on and when it goes off I hear a "bump" noise at the same time. HELP!!Thanks in advance.
Elder User
Joined: Dec 2006
Posts: 550
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From: Hillsboro, OR
Not too sure as I have not had this problem but from what I have read here, it sounds like your anti-lock controller is bad. Either that or it needs bleeding as well. You may want to do a search on the rear alti-lock to learn more about it until someone with more experience in these chimes in. Good luck.
Post Fiend
Joined: Jan 2005
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Here`s the thing. There is a RABS valve block on the inside of the left side frame rail about level with the driver`s feet. This has two solenoid operated valves. When the speed sensor detects that the rear brakes have locked one valve closes and prevents fluid from the master cylinder getting to the rear brakes. At the same time the other valve opens and allows fluid to flow from the rear brakes into a spring loaded piston, thereby releasing the brakes. Now if the valve was closed in the fluid feed it would not allow fluid to get to the rear brakes at all and even if it did and was not allowing it to return then opening the pipe at the master cylinder would not release the pressure. The same argument applies if the rear flex hose had collapsed internally. Opening a pipe at the master cyl. would not release the pressure. As you have tried another new master cyl. and it is just the same I can only think that your brake pedal is not returning far enough or there is something keeping the actuating rod too far extended. Just to check this out you could try putting a couple of washers between the booster and the master cyl.
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Senior User
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I figured out the problem. There is a pressure relief valve that screws into the rear port of the master cylinder. There is a small plastic valve that goes in before the relief valve is screwed in. I had that in backwards. I knew it had to be something easy since I got a second new master cylinder and it did the same thing. I also bled the RABS module but that didn't help. I didn't pay quite enough attention when I took it apart.
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