More Brake Woes- Help! - The 1947 - Present Chevrolet & GMC Truck Message Board Network

avejoe · 01-10-2005, 12:42 AM

More on my brake dilema...

Last weekend I had the rear drums turned. The guy said one was beyond spec and the other at spec (prior to turning). I talked him into turning them anyway.

Has anyone had brake performance issues after having their drums turned just a little beyond specs?

Could this be why I can't lock the rears up? Could this be affecting the prop. valve flow to the fronts?

AAAAAAAHHHHHHHHHHH!!!!!!!!

Thanks!

3 on the tree · 01-10-2005, 01:31 AM

The brake drums can oval out and break-metal too thin to withstand the pressure exerted by the brake shoes. If they are too thin, the wheel cylinders can extend out too far and blow out also.

4x4Poet · 01-10-2005, 01:59 AM

3 on the tree is right on (man

). At an AF base auto hobby shop, I once saw a drum that had been turned past its spec that was on display as a warning to others. The braking surface had a huge chunk missing from it like a shark had taken a bite out of it. Their display said "This is why we won't turn drums or rotors past their maximum spec allowed." (They also had displayed a destroyed rotor that had been turned to far. Wasn't pretty, either). Cast iron usually fractures when it breaks. Fractured drums provide little braking performance.

As to the proportioning valve problem, I doubt that the extra distance traveled by the rear wheel cylinders would cause front brake problems. Especially if you were able to reach expected braking pressures in the rear system. That would require a pressure check to verify that conclusion, though. In any case, you now know you must slap on new drums. Might as well wait until you do that before proceeding further just so you'll have eliminated one cause of poor braking performance before trouble shooting further.

The rears could conceivably not lock up because of the drums being at their max allowable diameter. That is a ways for wheel cylinder pistons to move. By the way, have you checked your adjusters? If they don't turn freely they won't take up the slack at the bottom as the shoes wear. The wheel cyl pistons take up slack at the top. If the adjusters don't move the shoes towards the drums as both wear, the shoes won't be in proper alignement with the drums and may not enable lock up. No matter, really, since you should install new shoes, springs, and adjusters before you install new drums.

If you have some spare change,

check out Praise Dyno Brake. Awesome componet upgrade using stock-sized stuff. Perhaps you noticed the discussion of PDB on this earlier thread.

Tx Firefighter · 01-10-2005, 08:28 AM

The drums aren't going to affect the stroke of the wheel cylinder or cause them to overextend. The shoes fits snugly against the drum if they're adjusted properly whether the drum is brand new or cut past it's limit. The excess slack is taken up by the adjuster at the bottom, not the stroke of the wheel cylinder at the top.

A little bit oversize isn't likely to cause any failure. A good bit over the limit might. It's a judgement call. A few thousandths isn't that critical.

Try readjusting your rear shoes again.

A little trick. Adjust the rear shoes till they just barely drag. Now, reach over with your hand and jerk the park brake cable where it comes out of the housing on the trailing arm. Now readjust the brakes. Jerk the cable again and readjust till they no longer change when you jerk the brake cable. By jerking the cable like that, it applies the park brake for a second and recenters the shoes in the drum.

A little bit of trivia that a lot of folks don't know, there are some vehicles that spec how many cracks are acceptable on a used drum and how big those cracks can be, and the drum still be reused.

Gordo · 01-10-2005, 06:15 PM

New shoes installed with "out of spec drums", only a small portion of the shoes actually makes contact with the drum, probably the middle of each shoe even when properly adjusted. Now when you apply more pressure the shoes are bending, trying to make full contact with the drum thus giving you a spongy, crappy pedal. Until you wear in those shoes to make 100% contact the brakes will work poorly. I would highly recomend new drums. Good brakes are #1, most anything else you can cheat on. It is also recomended that the drums be within .010 of each other after turning.

01-10-2005, 01:59 AM	#3
4x4Poet Registered User Join Date: Nov 2004 Location: "Under Montana skies." Posts: 1,836	3 on the tree is right on (man). At an AF base auto hobby shop, I once saw a drum that had been turned past its spec that was on display as a warning to others. The braking surface had a huge chunk missing from it like a shark had taken a bite out of it. Their display said "This is why we won't turn drums or rotors past their maximum spec allowed." (They also had displayed a destroyed rotor that had been turned to far. Wasn't pretty, either). Cast iron usually fractures when it breaks. Fractured drums provide little braking performance. As to the proportioning valve problem, I doubt that the extra distance traveled by the rear wheel cylinders would cause front brake problems. Especially if you were able to reach expected braking pressures in the rear system. That would require a pressure check to verify that conclusion, though. In any case, you now know you must slap on new drums. Might as well wait until you do that before proceeding further just so you'll have eliminated one cause of poor braking performance before trouble shooting further. The rears could conceivably not lock up because of the drums being at their max allowable diameter. That is a ways for wheel cylinder pistons to move. By the way, have you checked your adjusters? If they don't turn freely they won't take up the slack at the bottom as the shoes wear. The wheel cyl pistons take up slack at the top. If the adjusters don't move the shoes towards the drums as both wear, the shoes won't be in proper alignement with the drums and may not enable lock up. No matter, really, since you should install new shoes, springs, and adjusters before you install new drums. If you have some spare change, check out Praise Dyno Brake. Awesome componet upgrade using stock-sized stuff. Perhaps you noticed the discussion of PDB on this earlier thread. __________________ '71 GMC K20 Suburban, '71 GMC K10 Suburban, '72 Chevy C10 CST Suburban, '72 Chevy K20 clunker pickup. Last edited by 4x4Poet; 01-10-2005 at 02:12 AM.

01-10-2005, 08:28 AM	#4
Tx Firefighter Watch out for your cornhole ! Join Date: Jun 2003 Location: Azle, Texas Posts: 14,162	The drums aren't going to affect the stroke of the wheel cylinder or cause them to overextend. The shoes fits snugly against the drum if they're adjusted properly whether the drum is brand new or cut past it's limit. The excess slack is taken up by the adjuster at the bottom, not the stroke of the wheel cylinder at the top. A little bit oversize isn't likely to cause any failure. A good bit over the limit might. It's a judgement call. A few thousandths isn't that critical. Try readjusting your rear shoes again. A little trick. Adjust the rear shoes till they just barely drag. Now, reach over with your hand and jerk the park brake cable where it comes out of the housing on the trailing arm. Now readjust the brakes. Jerk the cable again and readjust till they no longer change when you jerk the brake cable. By jerking the cable like that, it applies the park brake for a second and recenters the shoes in the drum. A little bit of trivia that a lot of folks don't know, there are some vehicles that spec how many cracks are acceptable on a used drum and how big those cracks can be, and the drum still be reused. __________________ I'm on the Instagram- @Gearhead_Kevin Last edited by Tx Firefighter; 01-10-2005 at 08:32 AM.

01-10-2005, 06:15 PM	#5
Gordo Registered User Join Date: Jan 2001 Location: Killingworth, CT. USA Posts: 3,378	New shoes installed with "out of spec drums", only a small portion of the shoes actually makes contact with the drum, probably the middle of each shoe even when properly adjusted. Now when you apply more pressure the shoes are bending, trying to make full contact with the drum thus giving you a spongy, crappy pedal. Until you wear in those shoes to make 100% contact the brakes will work poorly. I would highly recomend new drums. Good brakes are #1, most anything else you can cheat on. It is also recomended that the drums be within .010 of each other after turning. __________________ 1971 C10 swb stepside 350/700R4/3.73posi (retired as of 4/22/03) 1998 S10 short bed 2002 S10 Blazer 1942 Oldsmobile 1958 Massey Harris Pony 1951 Wife Killingworth, Connecticut May those who love us, love us, any of those who do not love us, may God turn their hearts. And if God is unable to turn their hearts, may he turn their ankles so we may know them by their limping. A man who works with his hands is a laborer; a man who works with his hands and his brain is a craftsman; but a man who works with his hands and his brain and his heart is an artist.

01-10-2005, 12:42 AM	#1
avejoe Custer had a plan too. Join Date: Dec 2003 Location: No. California Posts: 344	More Brake Woes- Help! More on my brake dilema... Last weekend I had the rear drums turned. The guy said one was beyond spec and the other at spec (prior to turning). I talked him into turning them anyway. Has anyone had brake performance issues after having their drums turned just a little beyond specs? Could this be why I can't lock the rears up? Could this be affecting the prop. valve flow to the fronts? AAAAAAAHHHHHHHHHHH!!!!!!!! Thanks! __________________ 72 K15 GMC (aka Big White) Performance Rockers, Cab Corners, and Floors Fossil Fuel Glutton Neighborhood Nuisance 98 BMW 540i for Speed (Sold ) 05 Subaru Outback for mileage

01-10-2005, 01:31 AM	#2
3 on the tree Registered User Join Date: Oct 2004 Posts: 393	The brake drums can oval out and break-metal too thin to withstand the pressure exerted by the brake shoes. If they are too thin, the wheel cylinders can extend out too far and blow out also.



Register or Log In To remove these advertisements.

Register or Log In To remove these advertisements.