Riddle me this....

[wes beem]

71 Duster, 4 wheel power disc brakes. Was originally all drums.

The brakes (front and/or rear both) will RANDOMLY partially and or completely lock up in any combination while im driving and wont unlock unless i break a line on the master cylinder and let the pressure out.

Very intermittent. Doesn't happen every time.

 

[HemiSSDart]

Show us what master you have and if you have a valve in the system after the proportioning valve...

 

[Professor Fate]

And your distribution block and/or proportioning valve, and how it's plumbed.

 

[wes beem]

ill grab pics tomorrow, the master cylinder and booster are from PST. all brand new.

Front Disc Brake Conversion Kit

The solution for performance muscle car owners - PST's Disc Brake Conversion Kits.

p-s-t.com

 

[wes beem]

Show us what master you have and if you have a valve in the system after the proportioning valve...

no valve in the sytem, just the distribution block/proportioning valve thing all 4 brake lines go to.

 

[wes beem]

Decent shot of the master cylinder and booster. ill try to get one of the distribution block tomorrow.

 

[Syleng1]

That’s a master for disk drum combo.

 

[Dana67Dart]

My money is on the pushrod between mc and booster is not adjusted correctly. (Most likely too long) and it is putting the mc piston in just the wrong position to relieve the pressure from time to time

 

[wes beem]

My money is on the pushrod between mc and booster is not adjusted correctly. (Most likely too long) and it is putting the mc piston in just the wrong position to relieve the pressure from time to time

After talking to Cas at Doctor diff im thinking the same thing...this was my first time ever doing this and i didnt even know there was an adjustment in that. I just assembled it how it came in the box.

 

[Dana67Dart]

If the pushrod is too long it puts the piston in a position where it does not allow fluid to return the reservoirs.

There are procedures in the FSM but basically the the pushrod should be a few thou shorter than the distance inside the master cyl. The keeper inside the mc might be there too and it might also need to be removed.

Also there might be a residual pressure valve inside the forward most port on the master cyl. It would have been there from when the master was used on a disk drum setup.

 

[Projectile Dart]

Make sure your brake pedal rod is adjusted correctly. A buddy of mine converted from single din master cylinder to a double din and the shop that did the work left the rod too long (too tight) so as he would drive the brakes would lock themselves up in the backs and wouldn't release until you cracked the system to release the pressure

 

[Kern Dog]

That’s a master for disk drum combo.

You are right but I've also ran a disc/drum master with 4 wheel disc with no problems.
What bothers me is The way that it is mounted, he barely has room to put a battery in the car.
No offense to the OP but what a crappy design that is...A stock 73-76 A body setup would be a much cleaner, compact setup.

 

[wes beem]

You are right but I've also ran a disc/drum master with 4 wheel disc with no problems.
What bothers me is The way that it is mounted, he barely has room to put a battery in the car.
No offense to the OP but what a crappy design that is...A stock 73-76 A body setup would be a much cleaner, compact setup.

Ironically thats what i thought i was buying...

 

[TrailBeast]

You are right but I've also ran a disc/drum master with 4 wheel disc with no problems.
What bothers me is The way that it is mounted, he barely has room to put a battery in the car.
No offense to the OP but what a crappy design that is...A stock 73-76 A body setup would be a much cleaner, compact setup.

I’m running a disc/drum master with four wheel discs, and have for a couple of years with zero problems.
Even had to do a hard stop recently when someone pulled out in front of me crossways in the road and stopped.
I was doing 45 and really had to lay into it.

 

[wes beem]

I’m running a disc/drum master with four wheel discs, and have for a couple of years with zero problems.
Even had to do a hard stop recently when someone pulled out in front of me crossways in the road and stopped.
I was doing 45 and really had to lay into it.

the interesting thing is, when the brakes are working properly the car stops GOOD, maybe even better than some newer cars.

 

[Kern Dog]

That looks like a dual diaphragm design, effectively doubling the amount of boost over the same diameter single unit. The drawback is that they look as long as a 4 gallon bucket.

 

[Inertia]

Back the m/c mount nuts off, test .