I recently replaced the pistons and seals in the rear calipers on an XJS back axle. I noticed that the pedal travel seemed rather long after re-assembly, even after fully bleeding all the brakes.

On closer inspection the the rear calipers, I noticed that the caliper on one side brings the pads up tight to disc and they stay there. As you'd expect.

The other caliper seems to tighten up when you press on the pedal but when you relax the pressure, the piston retacts back into the caliper, leaving the pads very lose.

I've just taken the caliper off and checked it with an airline. Its still doing it.

Video is here:


In this video you can see when I let the air out, the piston goes backwards.

This is will obviously mean lots of pedal travel when you apply the brakes!

What's going on?

I'm going to pop the pistons out tomorrow. Maybe the seal got twisted up?

 

It doesn't look right to me either !. Best to strip right down and try again. Swap the seals over too and see what happens. Dis you check the external surface of the pistons ?. The pistons should have a nice smooth surface to allow them to move past the seal until the pads hit the disc, but with enough drag that the seal gets slightly distorted. Then. when pressure is relaxed, the seal relaxes moving the pad a touch off the disc.

Edit
Are these modern-type pistons with the seal in the cylinder, or the old type with the seal on the piston and a withdrawal clutch mechanism and rod assemble at the back?

 

New pistons and seals. Old School, just a piston and a square section seal...