I have a 1990 F150 that the rear brakes keep locking on...I have replaced the master cyl, Checked the rubber hose from frame to rearend and its fine..I can release a bleeder on the rear,and it will release the brakes...I can release rear line on the master cyl going to the rear brakes and it will release the brakes...Could the ABS Hdy Unit be causing this?? After the truck sits for a day of 2 the brakes will release again , then after i pump them a few times or drive it a little ways down the rd and using the brakes some, they grab and hold again untill i use a bleeder or the rear line on the master cyl...Also does that one way valve or what ever u call it, that screws into the master cyl , then the line screws into it ' Ever go bad ????

 

Yes, the HCV valve can be the problem, it was on my '93. Took it off and put a brake line on to jump around it. Took the HCV apart and found a lot of trash in it found one of the solenoids stuck, ad the other one full of trash Cleaned it and will put it back on as soon as I get a chance.

 

I second that, I have heard people report the same behavior and it was from the RABS valve. They are expensive. A cheaper alternative is to maybe get one from a junk yard and use that. Of course, there's a chance that one could be bad as well.

 

You guys talking about this part right here below ??

 

That is the one I was referring to, yes. They have issues. I'll be getting one from the junk yard tomorrow for a different problem I have.

 

A1 Cardone OE Replacement ABS Hydraulic Unit (Remanufactured) - JCWhitney

I was having rear lock-up issues as well on my '93. I'd get the stiff pedal after the first few brakes then BAM, lock up/wheel drag. I got the yellow ABS light and red Brake light every time.

This Hydraulic unit is a little on the costly side, but i've had it in 3 years now and have encountered no problems with it. Besides a salvage yard, you WON'T find these anywhere. Ford doesn't even sell them. I tried a few from the salvage yard prior to buying this one new and every one of them had a problem. (Gasket would blow out from dryrot...internal solenoid bad etc..). Save yourself a lot of misery and just get this RABS unit from JC Whitney.

 

Can they be bypassed ??

 

Yes, just disconnect the elec connector and then disconnect the two brake lines and connect them together. Bleed the rear system and you're good. But be forewarned, you will have a constant ABS light and rear brakes will operate like the older trucks- without ABS.

 

Ok Guys,
I step on the pedal and I feel pressure for a second and then it slowly goes to the floor. ABS light flashes 16 times (yeah right if it’s ok then why are you flashing)
I replaced front rotors, pads and calipers. Right caliper had small leak. Adjusted rear brakes.
Bled system with Mighty vac. Got air and lots of dirty fluid. Ran 3 quarts thru doing flush. Retested pedal the same. Was told ABS hydraulic assembly probably by-passing.
Replaced Hyd Assembly. Built pressure bleeder this time. Pushed 3 more quarts thru still soft pedal.
What the heck?

 

It would have been better to start a new thread for your problem rather than to have brought a six year old dead thread back to life.

You're problem is most likely the master cylinder is bypassing bad seals internally.

 

Agree. That is a typical symptom a bad master cylinder.

The OP “Checked the rubber hose from frame to rearend and its fine..I can release a bleeder on the rear,and it will release the brakes.”

I don’t know how he “looked at the hose“ and determined it was fine. I’d say the hose was swollen internally:restricting the flow of fluid back to the master cylinder. Obviously opening up the bleeder screw relieved the pressure. We don’t know if he ever found the problem since is last post was asking about bypassing the RABS and never came back. That was 6 years ago.

 

Sorry guys, that was my first post and really didn’t know the process.
I to had come to the determination that the seals are leaking on the master cylinder even though it was a new not reman and less than a year old.
I got a replacement yesterday and gonna try it today.
I’ll let you know.

 

I would not be surprised if a new or reman part has already failed.