Mk1 Rear Brake Caliper Piston won't wind in enough!

Hi folks,

I’m trying to fit new rear discs and pads to my Mk1 Berkeley.

All was going well, pistons wound in using adjustment screw, calipers off, caliper brackets off and discs replaced etc.

Between fitting the new brake pads, I had to put the job on hold and I reckon I must have accidentaly pressed the brake pedal whilst there were no pads installed in the calipers. When I later jacked up the car to fit the new pads the piston had come out, not all the way out, there were no fluids leaks, but the dust covers were a litlle extended. I’m now trying to wind the piston back in so I can fit the news pads, but it wont go in far enough. It also won’t go in of its own accord whilst turning the adjuster, I have to apply gentle pressure to the piston (hand pressure) whislt winding the adjuster to get it to go in. I can wind it back out again without any problems, but to get it to go back in, I have to apply a little pressure whilst turning the adjuster anti clockwise. Even using this method, I can’t get the piston in enough to fit the new pads, it needs to go in about another 5mm or so.

I have also loosened the handbrake off completely, but I still can’t get the pistons to go in enough. Should the aduster pull the piston back in of its own accord without any pressure applied?

I think the adjuster may have dislocated which is why it won’t wind in on its own, but why can I wind it out fine? Can I reset the adjuster in some way easily without taking the caliper to bits?

Or have I mullered the caliper/s and now looking at a costly mistake? The lid is off the reservior in case anyone points that one out.

Any advice greatly received, cheers.

 just done my calipers found same problem, it was a small amount of corrosion on the pistons under dust rubber,a repair kit required seals. dust. cover etc…after a clean up( do not put any pressure on the piston)Big Smile

Just had a look, but no corrosion to speak of.

I desided to wind the piston all the way out and remove it. The auto adjuster inside had come free of the piston. I wound the adjuster off the end of the adjusting screw inside the caliper, but how do I refit the auto adjuster back into the piston? It must be possible as MX5parts sell replacement pistons less the auto adjuster. Is it a G-Clamp job to get it back in and reseated before reinstalling it into the caliper and then hopefully winding the piston back in?

 just screw the piston back in and finish of with the adjuster do not use a G clamp as it can damage the adjuster screw,it should only be finger tight


I think you have miss-understood the problem.  As I read it, the adjuster has come out of the piston and out of the caliper, and the question is “How do you get the adjuster back into the piston before putting the piston back in the caliper”.  No doubt I’ll be corrected if wrong, in which case apologies in advance.




Have a read of these extracts from the FAQs—

 Quick update, I got the auto adjuster back into the piston sucessfully by putting it in a vice at work. The vice had protective jaws on so to not damage the piston or auto adjuster. Tightened up the vice gently and slowly making sure the auto aduster got pushed back into the piston squarely. You know when its fully home when you hear the C-spring locate in the machined out grove within the piston. All back together now and is 100%.

So for those of you who with rear calipers where you can wind the piston out OK, but cant wind it back in, its likely that the auto adjuster has dislocated from the piston. The reason you can still wind it out is beacuse the auto adjuster simply pushes against the back of the piston, but is not located inside the piston so when you try to wind the piston back in the auto adjuster winds in but the piston remains put. If you wind the piston all the way out and remove it, chances are the auto adjuster will still be sat in the caliper body on the end of the aduster spindle.

The pictures in the link below helped me out allot, figure M shows the auto adjuster and figure J shows how it shut locate in the piston.