Hi. My 1200 sedan that I just put on the road has new discs , calipers and master cylinder. New cylinders in the back. The car had been sitting for around 18 months with these new parts. Before registration I bleed the brakes till new fluid came thru all corners. All was working well for about 1 week. Then the pedal started going down with pressure on. Bleed the system again. Same thing. Bleed the system again. Same thing. I started with the master cylinder. Back passenger, back driver, front passenger, front driver. Same thing again. F#@ck. This is not a fun job in a dirt car park after rain in the middle of West End Brisbane. I did notice the last time that I could really hear the front passenger caliper working, but not so much the front drivers caliper. There is no leaks anywhere so I'm thinking the front drivers caliper might be sticking and causing the problem??? I have never had this much trouble bleeding brakes before so any help/suggestions if anyone else has had a similar problem would be greatly appreciated. It's not a boosted system too. Cheers...

I would think it is the seals in the master cylinder. But being a new item that's a little unlikely.

Otherwise I would have said you've got air in system but it sounds like you've definitely bled it. Is there a chance it could be the seals in the master cylinder? When they get old and perish they tend to allow pressure to bypass which makes the pedal slowly sink when you've applied the pressure.

Are you loosing brake fluid?

Not related to the sinking brake pedal, but did you make sure you put the coupe proportioning valve in your system when you went to coupe struts? (guessing that you've put in coupe struts or 120Y struts since sedans never came with discs up front)

Do you have discs up back or just the H145 diff with drums?

Hi. I bought the car a few months ago. It had bee sitting for 18 months. The master cylinder and calipers where replaced with new items about 6 months before that. So its all relatively new. It was working very good for any old brake set up for about a week before it started sinking. I did notice the reservoirs where not that tight today before bleeding the system again. So tightened them up. I have put a whole lite of fluid through the system at this stage. I have a feeling the front drivers calipers is sticking which wouldn't allow a proper bleed on that side. Its a funny one. I've had 6 1200's and Ive never had this happen. No leaks either.

Yeah be worthwhile to pull both front calipers off, pull the pistons out, give the bores and the pistons a light rub over with some steel wool soaked in brake fluid (try get the wool without the soap in it) to get rid of any rust etc. Clean everything with brake cleaner, then rub some fresh fluid on the seals, pistons and bores, re-assemble and chuck them back on the car.

Get some freeze spray or a butane can and spray the piston when you pull it out and put it in, makes your job a lot easier.

I would be getting a new master cylinder mate or having alook at that drivers side caliper

A stuck caliper cannot create a bleed problem

If the master cylinder was recently rebuilt , it Likely is the problem . It is not uncommon.

The master cylinder is new. I have a whole sunny front end in my other car so was going to swap that over soon. Just want to get the brakes functioning properly for now. It's a bit of fun in the cold round the city. Heel toeing and pumping the brakes. Good practice lol.. I think i might pull that caliper off and have a look...

definitely be leaking back through piston in master if you can find a leak elsewhere

just did mine again on rally car, was only a couple of years old and rally car is only driven maybe once a month

new masters are possibly not very good quality these days?

i think the bleeding process must work them pretty hard and thats what shags em, mine was working fine before i got new brake pads and bled the system, then couldn't get pedal same as you describe

Yes. Perhaps you misunderstood me. Many new/brand new "remanufactured" master cylinders are turning out defective.

Again, your caliper can't cause this problem unless it is leaking or hasn't been bled properly. If it is totally stuck 100% seized it won't cause a spongy pedal. Look elsewehere for the cause.