[Mk2] [NA] Rev 3 calipers on Rev 1 now binding

[tony67]

I got a set of rev 3 calipers from a fellow board member just this week. I have fitted them but now the brakes are binding. When I was fitting them I pushed the pistons back, they were stiff but moved all the way back in. I noticed today when I went to push both pistons back in together they wouldnt budge. If I push one in the other comes out and vice versa. If I immobilize one piston with a clamp and try push the other in it wont budge. To get both to go back in together I have to undo the bleed nipple and let the fluid escape that way. Its as if whatever is "upstream" of the caliper wont let the fluid back up into the reservoir. This is not normal to my knowledge. Im pretty sure its not the calipers, the pistons were stiff but not that bad and the pistons move freely one at a time. The slide pins have a nice smooth action too so its not them.
Im thinking when I push the brake the pistons come out and act on the pad/disc but when I release the brake for some reason the fluid cannot get back up to the reservoir and remains pressurized thus causing the brakes to bind.
Does anyone have any idea what could be causing this? In case its relevant the car is a rev 1 with rev 3 calipers non ABS model and seemed to be working fine till I changed the calipers.
Any help will be appreciated.
Tony

Edit I just remembered I have noticed a reduction in the amount of free play or slack in the brake pedal since doing the work. There is pretty much none before the brakes bite. I have found this

http://www.tegger.com/hondafaq/mastercy ... works.html

Im beginning to wonder if my master cylinder has either failed due to me bleeding it with the pumping method or needs adjusting to regain some freeplay.
Anyone know how to adjust the brake pushrod?

[going4the1]

There isn't anything wrong with your master cylinder. The fault is with the new calipers. I'm afraid that you will need to buy a refurb kit and swap the seals.

The problem is that the pistons develop a layer of crud which jams them against the seals. What tends to happen is that as pads wear more of the piston is exposed to the elements and corrosion builds up on the exposed surfaces (even though they are behind the rubber boot). If new pads are put on the car and pistons are pushed back then this layer of crud acts on the seals and jams the whole thing up.

[tony67]

OK 5 hours work today including driving 60 miles to get a salvaged master cylinder. I removed the master cylinder and attempted to push the pistons back in pairs to see if they would move the did and fluid came out of the 2 pipes that screw into the M/C. It was obvious at that point the problem was the M/C. I tried to disassemble the unit, got as far as removing the 1st piston. I removed the bolt underneath that retains the 2nd piston but the piston wouldn't come out. I done as the manual says and smacked it off a piece of wood.... no joy. I then tried to push it in and it wouldn't budge. I then realized it was seized and was a lost cause. I went and got a salvaged M/C and have just spent the last 2 hours fitting that and bleeding the whole system. Now she's perfect freewheels nicely and rolls to a stop without the binding feeling I was getting.
So anyone who has similar problems in future if the usual caliper overhaul doesn't work or isn't applicable consider your master cylinder. That link I posted above gives a good understanding of how a M/C works and helped me diagnose my problem and might help someone else out.
Tony