Rear brakes locking up!

My M code mach 1 locks both rear wheels under heavy or sudden braking. I have replaced the master cylinder, re-built front calipers, re-built proportioning valve, flushed brake lines and hoses and replaced rear wheel cylinders. The only thing is i fitted slightly smaller ones than the originals which are 29-32" with 7/8", which are not a great deal smaller. If anyone can offer any advice on my problem it would be much appreciated. I dont want to have to replace the wheel cylinders with the correct size only to find it hasnt fixed the problem.

Hey
Does it have power brakes?
Has it always been a front disk brake car?
Are the rear shoes rivitted or bonded?

Yes it has factory power brakes and it has bonded rear shoes

is this a new issue or is it a new car for you?
Just checking as if you are used to driving with a ABS equipped vehicle well....

When I hit the brakes earlier this year the back locked up and we did a little 2 or 4 wheel slide. I thought/think they may all do that. Manual ABS with th efoot was required

Also check the age of the rear tyres, make sure not old hard tyres.

I have heard from a few people that by fitting smaller wheel cylinders they will lock up but since they are just over 1mm smaller i dont think they would make any difference. When you say yours does it, does it do it all the time as mine doesnt matter what speed your doing i can get the back to lock up. I would find it hard to believe that its normal for it to do it. Has anyone else experienced this with there's? I also suspected my tyres so i tried some others which were new and bigger and did the same. Under normal braking there fine even at high speed but if i hit the pedal hard quickly they lock even for that split second whilst on the brake pedal. Normally when you hit the brakes hard its the fronts that will lock but i cant get mine to do it.

I see you are in Melbourne too so have you tried the Oz forum as well? I suspect it happens at most speeds as I was not travelling too fast just suddenly braked. maybe also proportioning valve in master cylinder has an impact?

Get 3/4 wheel cylinders
That is what Ford Aust did for XB/XC Falcons back in their day.

Thats smaller than what i have now, it would make it even worse.

Making the wheel cylinders smaller will not increase braking pressure.
Quite the opposite.
It will give a harder pedal with less travel and reduce rear lock-up.
It is the opposite to this:

"Improved Braking
The bottom line is, if the stopping power of a car needs improvement, or if there’s a need to reduce the pedal effort, several options are available: (1) Decrease the master cylinder bore size; (2) Increase the pedal ratio; (3) Increase the wheel cylinder bore size."

Thanks for the info. Can you answer this then. Why did they originaly come with 29/32" wheel cylinders which are much larger. Now that I have fitted 7/8" wheel cylinders which are smaller it hasn't made it better?

Also I have noticed that under normal braking they are very sensitive and do not need much pedal pressure to stop or slow down.

Believe what you want to believe.
I don't question the physics.
Google it and then tell me it is still wrong.........

Im not saying your wrong, im just asking why originally they came fitted with larger wheel cylinders?

No worries no offence taken.
Why did Leyland make the P76? Anyones guess really.
It may have been lots of factors like tyre technology improving or road surfaces changing their coefficient???
Maybe even different driving styles.
I don't know, but I do know, it's not the first time a car manufacturer has made design faults.
Look at all the vehicle recalls made today.