Ive just put 120y calipars on the front of the 1200 and after a few uses the fronts stick on. Pretty hard, cant push it and it feels like your always in to higher gear when driving. Soon as you crack a bleeder nipple it comes free again.

Calipars were cleaned with new seals installed, discs machined, new rubber brake lines. New pads, (even tried old pads to make sure they werent to think). Check valve removed. Still didnt work so i went to the extent of buying another brand new 120y master. Has plenty of free play.

What else? Really Really getting to me now.

do you know if the caliper was moving freely when you put it on? I think that the disc run off is what pushes the caliper open again, but it sounds like the check valve was left in, but that can't be the problem because it has been removed.
I think this might be a question for dattoman_1000

hey freak, have a read here .... seems to be an issue with using the 120y tandem cylinder

Ouch man, I feel your pain, same thing happend to me. A quick swap over the weekend became a month long mission with tons of frustrations.

When you say it has plenty of free play do you mean free play between the brake peddal and booster shaft or free play between the booster and master cylinder?

Make sure when the brake peddal is at its highest point its not appling any pressure on the brake system. Check the distance between the booster and MC by getting the brakes to bond and then losen the MC from the booster, if the brakes release you know where your problem is.

I can not stress enough how important it is to line the MC up with the booster. If it is ever so slightly skew you will experians the exact same problem. It took me the odd couple of weeks to figure it out. I simple added a washer between the MC and booster on the one side and it now works, I still have minor issues with bonding but nothing I can't live with for now.

I hope this helps and good luck.

Its just a non boosted master cylinder. Was running a brand new 1200 master cylinder with check valve removed. Thought it could be faulty wand went and bought a brand new non boosted 120y master cyl. Yes the brake pedal has free play. Its not pushing on the arm at all.

When using them its as if the pump up. First few uses is fine and then the pedal slowy gets stiffer and stiffer.

Freak, i have had exactly the same problem, as you can see from this post,
http://datsun1200.com/modules/newbb/v ... c_id=24132&forum=1#129523
but after taking more off the metal away from the mushroom head of the push rod, it looks like i have fixed my problem, i'm using a 13/16 sunny (booster type) as a non booster master, because the thickness of the head keeps the master cylinder from fully returning and stops it from releasing pressure, u need to grind the pushrod back so that it just past the convexed washer, does this make sense?, r u running a booster? i can take piccys to explain it better,

yeah non boosted 120y master. ill go have a quick look. A pic would be great.

Cheers
Dan