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intermittent brakes


johnnymac09

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Hey Guys,

I am looking for some suggestions. I am having intermittent brake failure, sometimes I will press on the brakes and there will be little to no pressure. If I pump the breaks it builds up the pressure and they are fine. It seems to get worse the longer I drive without breaking, like when I drive on the highway for 1/2 an hour and then go to break, nothing there. I have checked the fluid level and bleed the lines a couple of times. There are no leaks.

Recently I changed the breaks front and rear, pads, calipers, rotors, drums, wheel cylinders, springs, adjusters, flex lines and a couple hard lines. It was a complete overhaul. I didn't change the master cylinder, booster or the proportioning thingy on the frame. In the rear I used the bleeder screws that have a check valve built in (they came with the wheel cylinders and I thought they were neat, made it easy to bleed) do you think they might be a problem?

The truck is a 1989 Ranger 2.9L. The problem has been happening since, but has gotten progressively worse.

I am thinking it is the master cylinder, but am not sure how to check it. Any suggestions would be greatly appreciated, I have to fix this before I get in an accident.
 


gotmudd

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i would try a two person bleed starting at the passenger rear then driver rear then passenger front then driver front tire. sounds to me like you didn't get alll the air out of the system. replacing everything you will no doubt have alot of air trapped in system.
 

Ranger44

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You can check the master cylinder by using 2 solid brass plugs to plug the holes were the lines are. If you have a stiff pedal, then it's most likely good. Pump the brake pedal with the vehicle off till it is hard. Then turn the vehicle on with your foot on the pedal. If the pedal sinks a bit your vacuum booster is operating properly, and if it doesn't move, well it's going out. Also check for any leaks, around the wheels, master cylinder, etc. Maybe try and bleed the system again, and see if there is air getting in the system. Check the condition of the fluid also.....
 

AllanD

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Inconsistant pedal height can sometimes be caused by worn brake pad thrust surfaces on the '83-94 type brakes (where the pad "tab" rests in the caliper bracket that's cast integral with the steering knuckle)

However complete brake failure, pedal going to the floor but "pumping up"
is usually the master cylinder.

There is no "proportioning valve" this is an incorrect name for any of several different devices that may or may not be installed in a given vehicle's brake hydraulic system.

The commonest item MIScalled "proportioning valve" is a simple "pressure differential switch" (which lights the "Brake system failure lamp" on your instrument cluster... though how you could POSSIBLY fail to notice the pedal going straight to the floor without a distracting red light to tell you the pedal just went to the floor escapes all my attempts at a logical conclusion...

another such device which also does not "proportion" the brakes commonly used on GM vehicles is a residual pressure valve, which essentially delays the release of the rear brakes to enable smoother stops.

Your vehicle may be equipped with RABS which uses a control valve to release pressure on the rear brakes to prevent rear wheel brake lockup, but even a broken line in the rear brake circuit would leave you with SOME brakes.

IF you aren't losing fluid my bet is the master cylinder is worn.

they do wear out.

AD
 

johnnymac09

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Thanks for all the great suggestions, I will give it shot and see what I come up with.
 

johnnymac09

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Hello,

I plug the master cylinder with some brass plugs and stepped on the pedal, the pedal was rock hard. I pumped up the brakes and then turned on the truck and the pedal softened a bit. So, that tells me that the booster and master cylinder are good based on the suggestions made in this thread. I bled the break lines again and did get air out, but I had to disconnect the break lines to test the master cylinder, so that doesn't tell me if there was air before or not. I am still have the same issues. Should I replace the master cylinder anyways?
 

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