CB750F1 Rear Disc Brake

[Doswen]

The rear caliper piston won’t  retract leading to overheating.
I’ve been on this problem for a number of days now and can’t solve the problem - help please.
I’ve tried two different rebuilt callipers.
New HEL braided oil line with new banjos. The banjo bolts are shorter than the originals.
Tried two different seal kits in the master cylinder. Now had it apart 4 times.
The return hole is clear.
The only way the pistons will push back in is by undoing the bleed valve or the banjo at the master cylinder.
The brake works releases fine by hand but if it is run up and gets hot the brake binds badly and overheats.
To be honest I can’t see how the seals in the master cylinder release the pressure when they both face the same way and the cup on the end of the spring is over the outlet hole.
I don’t know what else to try.
I also rebuilt the front brakes (twin disc conversion) and have no problem with those.

[Johnwebley]

After you have fitted a new caliper seal and bled the system..
Operate the pedal like you are  braking. The piston should move out. Then as you release the pedal  it should retract slightly.

Check it moves freely

Sent from my SM-A750FN using Tapatalk

[Laverda Dave]

To add to John's comment, make sure the outside metal edge of the pad is clean with no rust as this can sometimes just cause enough friction between the pad and the inside face of the caliper body to prevent the pad from fully retracting.

[Moorey]

Is it a F1 seal kit as they can be very hard to find now.

[Doswen]

After you have fitted a new caliper seal and bled the system..
Operate the pedal like you are  braking. The piston should move out. Then as you release the pedal  it should retract slightly.

Check it moves freely

John, the piston moves out and retracts enough to free the disc when the wheel is turned by hand.
If the braking is under load with the engine running it will get progressively hotter until is completely locks up.

[Doswen]

To add to John's comment, make sure the outside metal edge of the pad is clean with no rust as this can sometimes just cause enough friction between the pad and the inside face of the caliper body to prevent the pad from fully retracting.

Morey, the pads are new and there is no rust. The bike has just undergone a 2 1/2 year refurb.

[Doswen]

Is it a F1 seal kit as they can be very hard to find now.

Dave, it is an F1 and yes the kit was difficult to find in the UK. I had to order the GL1000 kit From the USA and cut down the plunger thread. The kit is otherwise identical but I have also tried another set of seals from another F1 master cylinder and it’s no different. Still locks up under load.

[K2-K6]

It is "classically" a restriction of the return route into the mastercylinder.

If all the caliper works ok and is demonstrably free,  then it's just the fluid that's holding the pads from retracting.  From then on it'll heat fairly quickly.

Are you certain that you've seen completely clear passages within the mastercylinder? I'd be surprised if it's not within there.

[Moorey]

Is it a F1 seal kit as they can be very hard to find now.

Dave, it is an F1 and yes the kit was difficult to find in the UK. I had to order the GL1000 kit From the USA and cut down the plunger thread. The kit is otherwise identical but I have also tried another set of seals from another F1 master cylinder and it’s no different. Still locks up under load.

No idea how much you cut off the plunger thread but i had to cut 6mm off the GL plunger and it worked fine.

[Doswen]

It is "classically" a restriction of the return route into the mastercylinder.

If all the caliper works ok and is demonstrably free,  then it's just the fluid that's holding the pads from retracting.  From then on it'll heat fairly quickly.

Are you certain that you've seen completely clear passages within the mastercylinder? I'd be surprised if it's not within there.

I‘ve had a cocktail stick through the return hole and blown it with an airline. I’ve checked it 4 times now !
I just can’t work it out.

[Doswen]

Is it a F1 seal kit as they can be very hard to find now.

Dave, it is an F1 and yes the kit was difficult to find in the UK. I had to order the GL1000 kit From the USA and cut down the plunger thread. The kit is otherwise identical but I have also tried another set of seals from another F1 master cylinder and it’s no different. Still locks up under load.

No idea how much you cut off the plunger thread but i had to cut 6mm off the GL plunger and it worked fine.

I matched the thread length to the original.

[Bryanj]

Also classic of hardened crap in the caliper seal groove.
Ive had them where a dremel brush just polishes the crud, you need to use a dental pick and change the seal

[K2-K6]

If you put a clamp on one piston and squeeze it backwards into it's bore,  then does the other piston come out,  or does the fluid come back into the reservoir?

[Doswen]

Also classic of hardened crap in the caliper seal groove.
Ive had them where a dremel brush just polishes the crud, you need to use a dental pick and change the seal

Yes was cleaned out with a pick and yes had new seals. The seal isn’t leaking. Also fitted another rebuilt caliper also with new seal and had the same problem.
So I don’t think the caliper is the culprit but I don’t know what is.

[Doswen]

If you put a clamp on one piston and squeeze it backwards into it's bore,  then does the other piston come out,  or does the fluid come back into the reservoir?

Neither piston will noticeably push back. I have to open the bleed valve to get them to move.