912 Brake Caliper reassembly

Morrie · 12:00 PM

I need a little help. Brake system pressure helped me get the pistons loose from the first caliper. I cleaned the piston, replaced seal, and tried to reassemble. Tle piston slides in easily to a point then some sort of spring in the piston that goes over the center pin prevents it from going any further. I don't want to force it ... there has to be a trick.

Efrain68912 · 11-19-2001, 11:37 PM

The piston must be inserted at a certain angle; did you notice the diagonal marks across the face of the piston? The piston must be inserted at 30 degrees or so relative to the caliper. I'll double check, my shop manual is... for once, at the shop.

Efrain
68 912 Coupe

Morrie · 11-20-2001, 09:30 AM

Thank you for the suggestion! I have placed the piston into the bore and rotated it to the 20 degree point specified in the shop manual. The piston moved in easily without binding and I could feel it reach and pass over the new seal. Now, when I apply pressure, the "spring' in the center of the piston does not want to let it slide any further over the center pin in the bore. I can depress it about 1/4 of an inch, then it stops and bounces back on the spring to the original point. I am stumped and don't want to force the thing. Do I need to start it at exactlu 20 degrees? This doesn't seem to make sense as the center shaft isn't keyed. Does any of this make sense?

pokemon · 11-20-2001, 09:42 AM

The spring tension you feel is normal. Its air thats being compressed in a chamber. Its part of a system that mechanically prevents the caliper going back on its own, away from the brake pad. Lets say it takes up any play due to brake pad wear.
It is indeed a bit difficult to reassemble with this "spring" force working against us. Push it in and it comes out again on its own.
If I remember correctly I put the caliper in a vice a force it back slowly. Leave it like that for a while to let the air escape and it should remain in its vice position.
The piston must be inserted at 20 degrees from horizontal. At one side the piston has a lower section. That section should point in the opposite direction of the wheel rotation. Make sure you insert it at that angle because once inserted its impossible to rotate it.
Regards,
Patrick

Morrie · 11-20-2001, 11:58 AM

OK, I tried the vise trick. I compress down a quarter inch or so and it feels as if I've hit a hard bottom, but the piston is still protruding too far out of the cylinder. I have applied up to approx 150 lbs of presure on the piston and it will go no further still. It still will rock a very little bit from side to side so it is not jammed in at an angle. Still stumped......anyone???

pokemon · 11-21-2001, 12:20 PM

I don't know why it seems to hit on something...
Try again to put it in the vise and leave it overnight. In that time the enclosed air can escape and the piston should not move out. Also wipe everything dry inside with a paper cloth. The less oil the easier the air can escape.
Also make sure there no dirt or rust left on the piston and caliper. It may stuck on it as tolerances are quite tight on these parts.
Its three years ago I did repair the calipers and don't remember the details nor have I a spare one to try again.Anybody else?
Patrick

Morrie · 11-22-2001, 03:10 PM

I got it. Assembly lube on the center guide. Placed a large nut over the center stud of the piston to assure even pressure. Put a bar over the stud and applied clamp pressure and in went the piston. Should all be downhill from here. Just didn't want to force it.

Thanks for your patience with the rookie on the block. I really appreciate the support.

Best

Morrie
67 912
Bahama Yellow