I have had the entire brake system replaced but the problem is still the same. When you put on the brakes there is a sort of throbbing in the pedal and the brakes grab then let go and grab again. There is also an on and off loud squeal. I have had a separate mechanic check the parts to make sure they are new. The only thing he found was a loose hose on the brake proportioning valve. He fixed it and the car was alright for a few miles and now the problem is back. I have never dealt with anything like this. Does anyone have any idea what this could be?
The entire brake system replaced? Really? All four wheel calipers / slave cylinders, all four hoses, all pads and shoes, all drums and discs, all the brake lines? The proportioning valve, the master cylinder and brake booster? I don't imagine that it has ABS, does it? That must have cost a fortune. If any of those parts have not been replaced, the parts that have not been replaced should be the culprit. Key word should. But it seems like a typical warped disc rotor problem except for the noise. Tightening a hose on the proportioning valve would not fix this, even temporarily. BTW, they are steel lines connecting the proportioning valve, not rubber hoses. If a line was loose, it would have leaked a considerable amount of brake fluid. Something seems amiss with that repair. In any case I will ponder this question overnight and consult the oracle. If I come up with a theory, I will get back to you. You are welcome. Praise Octep. Amen. Unless both mechanics are lying all parts have been replaced. The rotors as well. There was some small leak at the brand new proportioning valve. No the car does not have ABS. Maybe the mechanic's attitude is correct and it's from Mars. And maybe it's just an adjustment they aren't doing. You might consider doing this: In a safe place, such as a big empty parking lot or a desolate stretch of dry road, get moving 25 - 30 mph. Apply the parking brake. Gently at first, then apply it with more force. Be careful that you don't lock the wheels - this could cause a spinout and is why you are doing this in a remote spot and on dry roads. If the pulsating and shaking persists, this indicates that the rear brakes are the problem. If not the fronts are the problem.
ABS system is causing the pulsing. If you want the problem to go away, find a brake specialist shop and have them do a vacuum bleed of the brake system using the correct ABS diagnostic tool and bleed procedure. When the air is out of the ABS controller, your pulsing will stop.
Pulsing in the pedal is usually 'warping' of the rotors caused by overheating of some sort. Could be cheap rotors, driving style, sticking caliper, bad hose, or a combination of those. Locking up is contamination of the drum brake shoes. Contamination is either brake fluid from the wheel cylinders (which you said 'entire brake system' was replaced so that should not be the problem) or 'rear end' lube. There are seals behind the axle shafts that go bad. Sometimes just the seals can be replaced, other times bearings and even axles need to be replaced.
