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MG MGA - Caliper pistons won't return

I think I may have screwed up, but am not sure; want to run it by you...
After rebuilding & re-installing the calipers, the front wheels turn ok, but if the brake is applied the pistons won't return that tiny amount to allow the wheels to turn freely; ie: they won't budge without manually pushing back the pistons a bit.
I think the problem is that I smeared red rubber grease on the square seals that surround the pistons, and the outside of the pistons, in order to facilitate installation of the pistons without thinking that the grease may undermine the square seal's ability to grip the piston as it flexes back, pulling the piston with it. I'm thinking I should've used brake fluid instead of the grease.
That sound like I've got the cause and effect thing down or is there another reason why the pistons won't retract?
Rick deOlazarra

Apply brakes, then release the pedal. Assume the brakes drag. Open a bleed nipple to relieve hydraulic pressure. If the wheels then turn freely, the problem is in the master cylinder. There have been some new master cylinders recently that have that problem. Also there needs to be a tiny bit of free play in the master cylinder push rod, and the master cylinder piston needs to return all the way to rest against the end plate.
Barney Gaylord


Try Barney's suggestion first.
You only have to open one bleed nipple to reduce all the pressure.
I agree that the pushrod clearance is the most likely problem.
However, if the brakes do not release when you open the bleed nipple then your explanation is quite possible.
Your statement needs some clarification though.
It is not that the seal cannot grip the piston when it is supposed to flex back, but it does not grip the piston as it moves forward and therefore there is no required seal distortion.
The seals should not be greased but assembled with brake fluid.
You may be able to get improvement by manually retracting the piston many times.
The fluid will eventually clean the grease from the critical area of the piston, but it would probably quicker to remove and clean off the grease.

M F Anderson

Barney; thanks for your input.
Implemented your suggestion and the pistons continued applying pressure, eliminating fault with the m/c.
Mick; thanks for the correct perspective on what's actually going on. Makes perfect sense, of course.
The brake and clutch push-rods were adjusted as per the manual.
Am SO looking forward to going through the motions of rebuilding the calipers again; especially the fun part with the dust seals. :)
Rick deOlazarra

Rick, I agree with all the above suggestions but have you checked the flexible brake pipes. If not new they could have expanded inwards and are now blocking the return flow. Pressure from the M/C will force brake fluid through but it can then be held at pressure in the calipers by the blocked pipe. Again releasing the bleed nipples in turn might point to this. I had this and replaced the calipers needlessly before I discovered the blocked flexibles. Good luck.
P N Tipping

Thanks for your thoughts, Pete; the brake hoses are all new though. Just about all of the hydraulics are new; m/c, push rods, forks, pedals welded & re-drilled, pipes, hoses, wheel cyls., & clutch slave. Calipers are original & getting rebuilt.
Rick deOlazarra

This thread was discussed between 27/06/2013 and 28/06/2013

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