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mysterious 740 T brake problem

posted by  Shady Parking on  Sat Jun 28 07:03 CST 2003
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Hi Swede, cc: invar

I took the car for a test drive this morning and examined it carefully for the first time (it is my wife's car). I noticed that the valve stem caps on the front wheel are melted, indicating high heat in the front. I then drove around town for a bit and checked brake function by lifting the front of the car off the ground. The wheels rotate with normal drag, but the rotors are badly scored and the brakes are squeaking.

I surmise that there are two pistons in the master cylinder, and that the front one is sticking when the car gets hot, causing the drag I am experiencing, and then the subsequent likely boiling of brake fluid and loss of braking power. I must confess an error here: a couple of months ago, I topped off my resevoir with DOT 3 instead of DOT 4 fluid, about 1/4 cup or so. I wonder if that could have caused the failure?

I wonder if you could expand on your previous comments below:

Swede4 wrote: "The brakes can bind for mechanical or hydraulic reasons. Mechanical ones include sticking pistons or calipers. Hydraulic reasons can only be if pressure builds in the system. Pressure can only build if fluid is not free to return to the reservoir when it expands because of heat. This can be caused by the piston not being able to return far enough to uncover the return orifice."

What "heat", the heat of normal engine operation? Would that be enough to cause a piston to stick, and then release when the car cools down?


Swede4 wrote: "This [the sticking piston] in turn could be caused by the vacuum assist valve still applying vacuum to the system."

What would cause the vacuum assist valve to apply pressure to the system once the brake pedal is released?

Swede4 wrote: "Try to make it happen again and open one brake bleeder to release pressure. If it goes away you are on the right track."

Do you mean open one bleeder and see if that caliper releases while the other side does not?

Thanks fellows. I'm really scratching my head on this one and don't want to replace the master cyclider unless I'm sure that's the problem.

SP






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