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FMJ Front Disc - One Side Locks up easily

Wolf Tickets

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Sep 10, 2020
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Location
Seattle
THE PROBLEM:

The right front wheel locks up first, and relatively easily, when braking. Under harder pedal pressure, the left front then locks up and the car yaws to the right.

ALSO: The brakes feel grabby SOMETIMES. Inching out of my slight downhill, gravel driveway is hard to do smoothly without a little lockup of that right front. I get the feeling that it's not always the same, though.

THE HISTORY:
Years ago I swapped the spindles from an Aspen onto my 69 Charger to get a cheap disc brake system. I didn't change master cyl at the time and they worked fine for years (though in hindsight I realize that there was no metering valve so the rear drums probably weren't doing anything after the swap, but thats all in the past).

Last year, I got a TON of work done and the shop replaced everything in the system. I've got all new parts. (combo valve, master cyl, booster, calipers, rotors, pads, hoses, hard lines). ALL of the rear brake parts are new as well, including the backing plates and hard lines/hoses.

Since then, I've had the right front caliper fail (in hindsight I assume this is from it doing all the work) and the right front rotor warp (same reason) and replaced both of those. I also replaced the left rotor thinking it might be faulty, but no change in effect.

I just got the car aligned and put new tires on and that didn't change anything.

I've done the following tests:

With car in the air, spin each wheel and see that they both stop evenly when pedal is pressed.

Neither side drags at all, wheels spin freely with car in air.

Good flow from both bleeder valves when bleeding (no air, and good blast of fluid when valve is opened with pressure on pedal)

I'd love to take a pressure reading at both sides, but I don't have the tools.

Any help would be great!!

THX!!

BEN
 
Pressure gauges are inexpensive. You simply remove the bleeder and put the gauge in its place. Sounds like that check is needed.
 
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Also check the caliper piston movement.
Thanks Robby. I ordered the pressure gauge so I'll have that later this week. As far as piston movement, would you just pull the caliper and observe it while someone presses the pedal? When i changed the pads last time, both Pistons compressed back in as expected, smoothly.

Thanks again for your help!!

Ben
 
Good point!! I'll get one and report the results.
OK! Quick shout out to Summit, ordered on saturday, ground shipped sunday, and I got the pressure gauge today.

The Readings make NO sense!! The passenger side reads about 600psi, that's the one that locks up easily. The drivers side reads 1500psi. So we've got low pressure on the side that I would expect to see the higher pressure on.

I bled the brakes, tested again, same results, and bled them again after removing the gauge (just to be sure that removing the bleeder didn't introduce a little bubble) and test drove. Same as ever, easily locks up the passenger side.

Thoughts?

THANKS!!!!

BEN
 
I'd rebuild the calipers. Sticking piston.
Thanks 66Satellite47. Would you rebuild rather than just get another re-built one from the parts store? I assume so since you'd KNOW that it was good inside, but I just wanted to confirm.

I did just replace from the parts store and it behaved EXACTLY the same way, so between that, and my pressure readings showing lower pressure on the side that's locking up, I figured the problem MUST be upstream in the combination valve. I guess I could see if the pressure at both front wheel ports on the combi valve are the same, but I would have to figure out how to hook up the gauge as it only has the bleeder hole adapters with it.

Thx for your help!!

BEN
 
Plumbed incorrectly. The front calipers should be fed from common source so both pressures should be the same
 
Plumbed incorrectly. The front calipers should be fed from common source so both pressures should be the same
Thanks Mike, that's an excellent point. There are two ports on the combination valve to go to the front, one to the back. Somewhere i read that the combination valve can either feed both sides or you can plug one of the holes and do a Tee to run to either side. Sounds like I could solve this buy doing that now.

Thx
Ben
 
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