Drum Brake Issues

[Chewievette]

Hi all, I recently bought a 1968 C-10 LWB and I've been driving it regularly for a couple weeks now but the other day I started noticing the smell of hot brakes, much more noticable in the cab than outside. If you touch the lug nuts the rear brakes are considerably hotter than the fronts but not so much to burn. Sometimes when starting from a stop it feels like much more power is required to get started than normal, usually this feeling is more noticable when changing directions for the first time, D to R or R to D. It does seem to be enough to stall the engine at slow enough speeds but that could be a totally different issue.

The most obvious conclusion is that the rear brakes are dragging, however I have taken all of the drums off and inspected the brakes thoroughly most everything inside looks brand new except some of the springs, linkages, and cylinders. I adjusted them to feel just a slight drag when turned but that has not helped.

The second conclusion was that the fronts were not working and the rears were doing all the work but when the front is in the air and the tires spun the brakes work fine. I have thought of cranking the rear adjusters in and driving a bit to test the fronts but I have not had the time yet and I'm not sure if thats a good way to test them.

I have a very short commute with few stops so this is not a critical issue but it could become one so I need to get it fixed soon. I am ordering a front disc conversion this weekend but I dont know how long that will take. I would like to try curing this problem before side stepping it ...especially if its a problem with the rears that I am keeping.

[geezer#99]

Check your emergency brake cable. Could be adjusted to tight causing drag.

[Chewievette]

Today I replaced the rear wheel cylinders, shoes, hardware, and added the self adjuster hardware that was missing. Then re-adjusted the rears and parking brake. Test drive showed not improvement.

I then made a few laps around the local parking lot and found that if I come to a stop then just let the truck sit for a minute it will slowly begin to roll again. Repeated stops of various intensities both fwd and rev shows the exact same response.

The only thing I have changed in the brake system since I bought it was the master cylinder and since it is under warranty I will replace it again tomorrow. After that I am not sure what to do. I suppose if the master cylinder does not fix the problem then I will wait and see if the prop valve and master from the disc conversion fixes the issue.

Is there anything else in the brake system that could be causing the issue? A check valve, restrictor, anything like that?

[cparman]

You might have a rear brake hose collapsed. IF the flex hose going to the rear brakes is restricted a bit, It might cause your troubles. The comment about the EBrake cable is another thought.

[geezer#99]

Are your brake shoes new? And are the shoes installed correctly? Short on the front!!

[87chevy.com]

Have you tried bleeding the system? Or did I miss that... if you havent, start with that and see what comes out.. maybe the front calipers are siezed or collapsed front caliper hose or something of that nature.. maybe thats why the fronts are a bit cooler..
clint

[Chewievette]

So far today I swapped the shoes around so the short pads are in the front and the longs in the rear. I honestly dont see why it matters, anyone care to shed some light? I also swapped out the master cylinder, re-bled the entire system and of course readjusted the rears again.

No fix.

I'm going to disable the parking brake just to see if theres a problem there, even though I'm sure its not exactly working since it wont hold the truck on a hill at all. Then I'll change the rear brake hose and see if anyone in the area has a prop valve for cheap. If that doesnt fix it I'm tempted to disconnect the rear brakes entirely until the conversion kit arrives.

[wiggins]

I wonder if those drums have been resurfaced to thin? and that is making them get to hot.

[Chewievette]

Parking brake disabled and the adjusters turned until slightly slack; the pedal sinks a few inches before activating then seems to brake normally (as good as one would expect from fronts only) No sign of dragging. After driving around for 10-15miles the rears are starting to smell again so the adjusters have compensated properly and again are beginning to drag. I didnt have time to replace the rear flex line so I'll do that tomorrow. No one on the planet seems to have a four wheel drum prop valve so thats not going to happen.

I can live with this until I get the front conversion kit installed. I've decided to order the stainless brake line kit from LMC and just replace everything in the entire brake system. If I ever find the problem I'll let you guys know.

[Chewievette]

Went for another long drive, the adjusters have brought the shoes back into contact with the drum and they are again dragging.

It was proposed to me that maybe the fronts arent working properly but I dont know how that can happen when they seem to be working fine after the rears were adjusted in... any thoughts? I'm also confused that if the rears are working as they are supposed to and the fronts are not then why do the rear brakes lock up after I stop?

I also started having problems with what seems to be vapor lock today, which is the first time I've driven long distance when its over 70 degrees. But thats for a different forum. I'm just getting annoyed and need to vent a little...

Thanks for the advice guys, I'm going to let it (and me) cool off for a little while then I'll go get that flex line swapped out. After that I am totally out of parts to replace in the brake system until my disc kit shows up.

[Rooster's 67]

Our trucks use a self energizing, or duo-servo drum brake system. A quick google search of duo-servo brake systems will probably find you more info than you really want.

On to the basics, the front or primary brake shoe in our drum brake systems is shorter so that under braking the front shoe becomes a fulcrum in direction of rotation, reacting on the adjuster and pushing on the rear or secondary shoe with greater force. The hydraulic system is augmented in this manner. the rear shoe is kept stationary by the anchor pin at top above the cylinder.

[Chewievette]

Ok, now I'm confused...

The old shoes that I took off and the new shoes that I put on are the exact same physical size and shape with no significant differences at all. Only the pads are smaller on one pair vs the other, a few square inches at the most. I understand the dynamics and physics, or at least I thought I did, behind the drum brake but I am totally confused why one shoe being slightly larger would have a significant effect on the system.

[dznucks]

you said you bled the brake system. Did you notice the rear brakes bleeding slower than the fronts?

Its already been mentioned, but it sounds like a collapsed (or collapsing) rear brake hose.

[Rooster's 67]

like i said do a google search for duo-servo brake system and you will find a page that will probably describe it better than I.

from classictrucks.com website:

"As the vehicle moves forward, the self-energizing force is greater at the rear shoe. To keep lining wear even, the rear shoe's lining is longer and sometimes of different composition than the forward shoe. With a shorter lining on the primary or forward-facing shoe, self-energizing force is slightly less when the vehicle moves backward. Since the vehicle does not back up at high speeds, braking is sufficient in either direction."

or try this one:http://www.know-yourcar.com/drum-brakes.html

[CEE1NG_RED]

any updates?

[MADMIKE]

Originally Posted by Chewievette

The only thing I have changed in the brake system since I bought it was the master cylinder and since it is under warranty I will replace it again tomorrow....

Is there anything else in the brake system that could be causing the issue? A check valve, restrictor, anything like that?

What brand and part number did use for a replacement MC?
Did you check the pushrod length vs the pushrod hole size?
A lot of the GM MCs have very close but different pushrod lengths. Drum brake trucks use a 1.64" rod but the Disc brake trucks use a 1.63" clearance. .01" difference of too long will cause the brakes to drag.

I would pull the MC off the firewall and measure its pushrod hole depth, and then compare how long the pushrod is sticking out of the firewall/booster.

Check out the instructions under "Brake Booster Push Rod Adjustment" Although it's a Mustang site these instructions are applicable to any vehicle.
http://www.maximummotorsports.com/co.../MMBAK-1r1.pdf