Hello forum,

I have an 83 FB where I had the rear axle swapped out to get 4 wheel disc brakes. I am trying to replace the brake pads myself after doing a brake fluid flush. The rear drivers side piston will not compress even with a tool. I did see the piston extend out while I was doing the flush, so I do not think it is seized. (I had the rx at a track and worked the brakes really hard, hence the flush/ pads). The back right piston is more compressed but does not seem to go flush like the front pistons did with the compression tool. Front pistons compressed flush easy-peasy. I tried opening the bleed valve with the the tool providing pressure and still no movement. I have put as much force on it as the tool can handle. Any ideas or suggestions?

Much appreciated!

Chris

 

What kind of tool are you using?

Last time I did rear brakes the cube tool at the end of a ratchet wasn't cutting it, and I ended up using a threaded compressor like this:

 

I have tried a couple of tools. the first was an amazon favorite that had two plates and a ratchet handle that spread them. This worked well for the front pistons but did not go narrow enough to fit in the back calipers. So now I am using a single plate screw. Note that I took off the **** and and using a ratchet on the underlying bolt head to get more leverage. Looks like a crappy version of what you used. Do I need to worry about using too much force? I have been careful to keep pressure aligned with the piston orientation.

 

the rear pistons thread in, so you need to turn them.

 

^what he said. You need something like the pic I posted with 2 pins to fit into the cut outs on the piston.

 

Yep. Well that totally explains it. Just watched some vids on how to do it and ordered an appropriate toolkit. Thank you!!!

 

I always though that was weird. So when we depress the brake pedal, does the piston rotate a bit as it extends (albeit, like a millimeter, right)?

 

I am going to do a cycle to see if I can see the rotation for curiosity's sake

 

I googled and found the rotation is required because we have the ebrake mechanism integrated into our rear calipers. Here's an FD rear caliper exploded view for rebuilding, cribbed from

https://www.rx7club.com/3rd-generati...aliper-758710/

So you have to rotate the piston back in due to the threaded ebrake adjustment part #11, and that assembly auto adjusts for the ebrake.

 

It does makes sense that a screw mechanism should allow the ebrake cable to advance the piston without the need for brake fluid pressure...

 

The emergency brake mechanism is actually a cammig lever that pushes against a stack of bent washers that form a heavy spring action inside the center of the piston bore. Each time the parking brake is pulled, it cams this stack of washers against the piston which applies hydraulic pressure to the pads, holding the car in place.

Many mechanics pull the big rubber dust boot off, glance at the mechanism, and give up on that portion of the rebuild, because it takes a vice and some ingenuity to compress the stack and pull out the actuator rod. Problem is, that actuator rod has its own Viton o-ring to seal it under tremendous hydraulic pressure, and when it fails, the boot fills with brake fluid and leaks.

Rotation of the piston during compression acts against that cam lever and the washer stack, otherwise you wouldn't be able to compress it all by hand. The front piston are easy by comparison. You didn't damage it by NOT rotation it, but you also didn't get it to compress, either.

 

Another thing about the rear brake pads, the piston side will have two small notches that go into the corresponding area in the piston. i think this stop the piston from rotating as the pads wear, allowing the adjustment mechanism to work properly.

And yes, rebuilding the rear calipers is a bit of a pain. Take some tricks to get it all back together.

Maybe the pain to rebuild calipers are also why you can't find them anymore!

 

there is a big rebuilder in the US, and they moved factories, and when they moved they lost all of their cores.

 

I'm not necessarily recommending this, but after bending my threaded compressor tool without budging the cylinder I resorted to installing the cube tool on my impact gun.





It worked.

 

Also, I've noticed a few ebay posts for rebuilt FB rear calipers lately, don't know about quality though and they seem to have the left driver's side only:

https://www.ebay.com/itm/38417993746...wAAOSw6qpeqIhg

https://www.ebay.com/itm/28408853262...8AAOSwWSZfuB5U

 

I had ran into the same issue when swapping my rear pads thought it needed to be pushed in but after some reading I learned the piston needed to be turned in