Rear Brakes locking up

[ro23dart]

Took my 72 Demon out for a trip to the gas station and to do a little shakedown today. Brakes seemed fine at lower speeds but once i needed to slow down faster the rear brakes locked up on me pretty easily. It has drums on all four corners. I checked the parking brake (which works great by the way) to see if it was a little tight but with it out there seemed to be plenty of slack in the cable. My next step is to check the adjustment on the rear. I just wanted to get any ideas/opinions that on anything else i should check. The shoes seemed to be right, little one on the front, big on the back. Could something as simple as being adjusted out too far cause this issue?

 

[JBurch]

It's a good idea to go around all four corners to check and readjust; question I have for you is; have you just done a bunch of brake work?? Master cylinder, wheel cylinders? If so, is the master cylinder really a drum brake MC, and what size are the wheel cylinders? You're dealing with hydraulics, force over area, if the wheel cylinders are a larger diameter than original........even though parts book says it's the right one.....the larger surface area of the piston will generate more force on the shoes and cause lock up.

It's a long shot to be sure, if you haven't done any brake work forget what I said; if you have it gives you something to think about.

Good luck, be safe.

 

[ro23dart]

Oooh, thats a good point. I did just replace the rear wheel cylinders but i didn't have any brakes before so i have no frame of reference. Luckily i didnt throw out the old cylinders so i will check them against the new and see if there is any difference. Thanks!

 

[TrailBeast]

Check the adjustments first, as loosely adjusted brakes can cause the shoes to "cam in" and lock up.

 

[toolmanmike]

Check closely for wheel cylinder leaks on the wheel that is locking up. If it is a rear wheel you may have a slight axle seal leaking also. Either will cause the brakes to lock up and the tire to skid.

 

[ro23dart]

It's a good idea to go around all four corners to check and readjust; question I have for you is; have you just done a bunch of brake work?? Master cylinder, wheel cylinders? If so, is the master cylinder really a drum brake MC, and what size are the wheel cylinders? You're dealing with hydraulics, force over area, if the wheel cylinders are a larger diameter than original........even though parts book says it's the right one.....the larger surface area of the piston will generate more force on the shoes and cause lock up.

It's a long shot to be sure, if you haven't done any brake work forget what I said; if you have it gives you something to think about.

Good luck, be safe.

I re-adjusted the brakes which helped a little bit. But i think you may be on to something. The replacement wheel cylinders are 33597 at autozone and say they are 15/16" bore. The casting number i pulled off the old ones is 3461776 and when i cross reference that it comes up with 13/16" wheel cylinders. I am thinking that that extra 2 16ths could make a huge bit of difference. I was also wondering about my MC. I see some kinda confusing information on it and want to know what kind i have. I know this car has been messed around with a bit. Here are pictures of my ratty MC. I have read that this doesnt mean that it is for front disc brakes as long as it doesnt have the RPV in it.

 

[JBurch]

Bendix, used to offer a wheel cylinder rebuild kit for the 13/16 cylinders, might still be able to get a kit, might be a good idea so long as the cylinders you have now are not pitted. Some thing to think about.

I just did a quick number crunch, and the 15/16 wheel cylinders add just shy of 25% increase in piston area over the 13/16 wheel cylinder; about the only way to make the 15/16 inch cylinders work is going to be an adjustable proportioning valve on the rear brakes; OR try a set of 7/8 inch dia cylinders.

Good luck!!