1971 340 Cuda, rear brake cylinders.

[Manuel]

Hi,

Car in the shop again due to rear brakes comming on stronger than forward ones. 
( Fronts are disc type)

Mechanic would like to know if the rear brake cylinders diameter should be 13/16. 
That is what it has. 

It would be great if you can send a Pic and P/N of the correct rear break cylinder units. 

Thanks

[anlauto]

Are the two lines at the master cylinder hooked up correctly and not reversed ? Not easy to do, but possible if someone made the lines 

[moparroy]

'71 Service manual lists 15/16 for all Baracuda models. 13/16 is only listed for 6 cyl Valiant. But I doubt that is your problem - larger diameter with same PSI fluid pressure will only increase the pressure applied to the shoes.
According to the service manual premature rear wheel slide most likely caused by a proportioning valve issue.
Does it have the proper prop and metering valves installed?
The '71 Dodge SM is under resources on this site - I was looking at the '71 PLymouth book but should be same

[ph23vo]

I USED 3/4 REAR CYLINDERS TO STOP THE REAR LOCKUP ISSUE..EVERY E BODY WITH ORIG BRAKES HAS IT DUE TO SHORT REAR AND NO WEIGHT

[Manuel]

Very useful comments, I have forwarded them to the mechanic.

Believe that the part we ordered and installed was a proportioning valve.   Will check on the metering valves if that is a different unit ?

[moparroy]

There are combination valves that do both - as is the case in my '74.
I was under the impression that the original prop valves are no longer available so wondering if there may be a calibration involved. The '71 FSM seems to have two different figures on the prop valve - it says
TESTING PROPORTIONING VAL VE
When a premature rear wheel slide is obtained on brake application, it usually is an indication that the fluid pressure to the rear brakes is above the 50% reduction ratio for the rear line pressure and that a malfunction has occurred...

Then under testing the prop valve it says:
(2) Have a helper exert pressure on brake pedal (holding pressure). Obtain a reading on master cylinder output of approximately 500 p.s.i. (3) While pressure is being held as above, reading on the valve outlet should be 360-405

While to me is more like a 20-30% reduction...?

Metering valve seems to be more to do with pressure applied to front but I suppose if it is blocked...

[Bullitt-]

I'm reminded of a recent issue one of our members that had the parking brake adjusted to tight & the car would barely move...
I wonder if something similar could cause early lock up....