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MG MGA - Caliper pistons won't release enough

I rebuilt the calipers with new seals, dust covers & pistons.
When I push on the brake, the car stops as expected. After releasing the brakes, the caliper pistons do not retract enough; ie: they're still pressing against the pads though not as much as when directly applying the brake pedal.
With the front jacked up and applying the brakes and releasing, I can, with a bit of heft, turn the front wheels manually, The pistons remain in contact with the pads for quite some time before releasing any of their pressure and when they do, it's by a very small margin; ie: the front brakes still drag, but not as much. If I re-apply the brakes, the pressure once again is quite strong and remains so for quite a spell as before. While driving, it's as though I'm applying very gentle pressure on the brakes.
At the same time the rear brakes (which have been adjusted to where one more clockwise click on the adjuster will stop the wheels from turning manually) are not affected; they operate normally..
Hydraulic system's all new, including an AP Caparo m/c.
After rebuilding and re-installing the calipers, I pressure bled the system using the Motive pressure bleeder (using 10-15 psi.). Is that enough pressure to push the pistons against the pads?
I've no idea what's causing the problem.
Any thoughts? Thanks.
Rick deOlazarra

Check Barney's site, When this happens open a bleeder on the wheel and see if it makes a difference. If it does it's the master cylinder.
gary starr

If opening a front bleeder releases only one wheel, then it's a collapsed brake hose on that side. It could be happening on both sides, very common with old hoses.

If opening a rear bleeder releases both front wheels, it is definitely a master cylinder problem. Perhaps the brake master pushrod is adjusted a bit too long, not allowing the piston to return fully to rest against the end plate.

Master cylinder pushrods need to be adjusted for "minimal freeplay", not interference fit. Brake and clutch pedals should have return springs fitted above the driver's toes. You should be able to press a brake of clutch pedal 1/8-inch minimum (up to 1/2-inch maximum) with one finger before you feel resistance of the master piston motion.
Barney Gaylord

First off, thanks for the help...
I opened one of the rear wheel bleeders while the pistons were exerting their pressure on the pads. When I did, a small amount of fluid crept into the attached clear hose and the pistons released sufficiently to allow wheel rotation by hand; still dragging somewhat, but not anything like what they were before opening the bleeder.
So, here we go again; modifying new parts to get them to work right...
Barney, if you're there; on your site, I read what appears to be the fix for the problem I'm having, involving drilling a small hole in the master cylinder valve body. (Correct me if this solution doesn't fit the problem, but I think I'm on the right page.)
My understanding is that the hole should be drilled so that, when all is assembled, it's location is lined up somewhere within the inner diameter (center hole) of the valve washer behind it.
Assuming I have that right, I'm not clear on the meaning of the hole needing to be drilled "downstream from the edge of the valve cup". Perhaps saying it in a different way would help me gain a clearer mental picture of the hole placement relative to the valve cup. Thanks.

Rick deOlazarra

That note (and web page) was in reference to the TRW/Lucas replacement master cylinder, which retains to too much residual pressure making the brakes drag. The new AP Caparo m/c is not known to have this problem (unless you are in the process of discovering a new faulty part).

before you do anything to the master cylinder, do one more test. Pump and release brake pedal to make front brakes drag. Then loosen the brake pipe fitting at the master cylinder outlet port. If that relieves the brake dragging problem, then you know the issue is in the master cylinder. If ti does not relieve the dragging, then open the bleed nipple at the wheel. If that does relieve the dragging, then you know there is some obstruction in the hydraulic line between the master cylinder and wheel.

The note about drilling the small hole in the master cylinder brake side non-return valve is here:

The little number 2 in the diagram is pointing at the location to drill the hole This is downstream of the rubber check cup seal (which is inside of the metal shell when assembled), and inside of the rubber valve washer.
Barney Gaylord

For what it's worth, I have the same problem and I'm using the Caparo M/C. However, I'm not ready to chalk it up to the M/C yet. I'm using silicone fluid and still have not been able to bleed all of the air out of the system. I've suspected that once operating and ambient temps come up that air that is still in the system is expanding from the heat and putting pressure in the system.

If it indeed turns out to be the m/c, this will be the 2nd major problem I've had with the Caparo unit.

--Jack Morris
JM Morris

A point of information - the original MGA master cylinder piston does have a small relief hole drilled in it to relieve the pressure when the pedal is released. These original pistons can easily accumulate muck and decay over a couple of decades, blocking this small hole, resulting in the brakes staying hard on after a couple of pushes of the pedal.
D Brown

This thread was discussed between 03/09/2013 and 11/09/2013

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