wierd "brake" problem... suggestions?

car; 1998 Grand voyager, v-6, 3.0L
Problem: after (very roughly) a half hour of driving, the brake starts to grab (as if I'd lightly pressed it) and slow/stop the car.
if I stop and pump the brake pedal a few times, it releases and lets me go for a few minutes, then starts up again.
If I then shut off the engine and restart, I get some more time before it happens again.
(If it's a completely cold startup, I get that very roughly half hour).
The shop replaced the front brake pads which were kind of ok, but pretty long in the tooth, and also a caliper.
Same thing happened.
They're guessing at straws now, and I'd rather come up with some focused ideas rather than go through each piece being replaced..
Any suggestions? Thanks
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danny burstein wrote:

Do you even have to touch the brake pedal for this to start to occur?
if not i would suggest you check for free play between the brake pedal pushrod and the master cylinder. you should be able to wiggle the pedal a small amount (up /down) like 1/8 inch before you "feel" the pushrod contact the master cylinder piston.
if this has no clearance it can hold residual brake pressure on and when the front calipers warm up the pressures could rise.
or you power brake booster could be faulty. and self applying.. disconnect the vacuum line to it (plug it) go for a drive (CAREFULLY CAUSE THE BRAKES WILL BE VERY HARD)and see if if they come back on.

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writes:

Nope. I first notice it as my car slows down... (or, if I've got the cruise control running, I'll see the RPMs go up or hear the engine laboring).

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