41 Chev Pass.-Front brakes wont release

41 Chevy special deluxe. Drum brakes on all four wheels.
I've converted to a dual (chamber) master cylinder.
Using a new (not rebuilt) master cylinder for a 68 mustang front drum, manual. This one:
http://www.autozone.com/autozone/pa...9q6Z8kn47?itemIdentifier=248424_0_0_3482,5662

Tried to take it for a test drive and made it out of the driveway into the middle of the street and the car wouldn't roll. Turns out the front brakes were locked solid.

The pedal starts off soft. I need to pump it a second time to get real pressure.
Once I do that, the front brakes lock solid and won't release.
(Rear brakes seem to be working fine.)
If I let some fluid out of the bleeder on one front wheel, both wheels release.

I have at least 1/4 freeplay in the MC pushrod.

All new wheel cylinders.
All new rubber hoses.
All old brake lines were removed and cleaned with brake cleaner and blown out with compressed air.

Any help would be really appreciated.
Thanks.
Joe

 

check to make sure the M/C pushrod has a small amount of clearance (1/16") when the pedal is released.

If the piston in the M/C isn't fully returning a port in ther M/C bore gets blocked and the condition you describe can result.


Ray

 

Thanks Ray, I forgot to mention that. I've updated my original post.

 

it sounds like there is a residual check valve in that master cylinder. you can take it apart and remove it. it looks like the top of a salt shaker. to first diagnose this, try switching the brake lines on the m/c. if the rears start to lock up, thats your problem.

 

Disc brake front or drum?

 

man, it's too early this monday morning. my brain needs more caffeine. forget the check valve, but switching the lines would tell you that the problem in surely in the m/c

 

Drum.

 

Removing the residual valves in that cylinder is not an easy task, they are under the brass seats inside the pressure ports.

Let's verify that this is residual valve problem before we start making changes to the system.

Normally a small amount of fluid should come out when yo crack the bleeder, but not a jet. 10 psi is almost nothing in a brake system.

If you push on the brakes and build up pressure in the system until you get the front brakes to lock up as before, release the bleed screw with no pressure on the brake pedal, if you see a short jet of fluid come out of the bleed screw and then the front wheel should release. This means it is a residual pressure problem.


I suspect that the master cylinder is improperly assembled. Autozone is good about returns. Swap it with another unit before you disassemble or modify things.

 

Thanks, I'll give that a try.

 

Yes, I would suspect a internal MC problem also.

 

Took the mc back to Autozone. They had to order one. Will be in Thursday after 3.