iDRY Vacuum Kilns

Sponsors:

'72 GMC6500 brakes help!

Started by BOBWOOD, July 22, 2013, 08:21:33 PM

Previous topic - Next topic

0 Members and 1 Guest are viewing this topic.

BOBWOOD

Hello all...I just got a good deal on a 1972 GMC 6500 with dump box...think it has a 366cui engine...I'm gonna use it for road construction/plowing and for hauling logs/slabs etc. around the property...the dump box does not go all the way to the cab so perhaps I will utilize this area to mount a knuckleboom loader some day...the unit seems solid and easy to work on... but the brakes don't work and I'm not familiar with the system which seems to utilize some sort of vacuum assist. The fellow I bought it from said the ps front slave cylinder/s were leaking and were the problem...but there is no indication of any leaking so I'm thinking either most of the slave cylinders are just kinda seized up from sitting around not being used or perhaps its an issue with the assist system. Any help would be appreciated as I have no experience with this type of system. Thanks BOB

thecfarm

Can't help ya BOB. But that loader sounds like a great idea.
Model 6020-20hp Manual Thomas bandsaw,TC40A 4wd 40 hp New Holland tractor, 450 Norse Winch, Heatmor 400 OWB,YCC 1978-79

snowstorm

if they are hydraulic brakes them yes it has a brake booster. sorta of the same as your pickup only bigger. unless the brakes are boosted off the power steering pump. take some pictures then we can tell what you have. 

jdament

Worked on those trucks a lot when I was a heavy truck tech.  It has a vacuum booster under the drivers step, if not there the passenger step, I get them confused with fords and IH's. The line from the master cyl. Goes to it.  The booster kinda has a master cylinder vacuum booster all in one.  These systems are the biggest pain to bleed out.  You pretty much need a power bleeder.  First see if the brakes are adjusted up.  That is also a pain because if it sat I'm sure the adjusters are siezed.  My experience if the booster looks old it is junk.  Getting these brake systems to work good can require a lot of work and money.  And if you just want to see if the master is even good crack the bleeder on the booster if you can.  Start there and hopfully this was some help.

sandhills

jd has it right (at least on our '68 chevy) the booster is under the drivers side along the frame, a lot of times it's just cheaper to replace the wheel cylinders then messing around trying to rebuild them.  Most of the adjusters are pretty easy to free up with a little time and penetrating oil, but also everything takes money these days  ::).

John Woodworth

Don't gamble with brakes, replace now for your sake aswell as anyone who could be involved in a brake failure accident, the booster will be your biggest cost, replace the wheel cyls. and master cyl. you will need a power bleeder and start with the booster first,L/F, R/F, L/R, R/R in sequence, unless you get it properly bled chances are the brakes will lock up when you step on brakes ond you will have to bleed off the perssure on the drum locked up to move.
Two Garret 21 skidders, Garret 10 skidder, 580 Case Backhoe, Mobile Dimension sawmill, 066, 046 mag, 044, 036mag, 034, 056 mag, 075, 026, lewis winch

sandhills

John could you explain your sequence of bleeding?  I was always told the opposite, start farthest from the master cylinder and work towards it, I'd appreciate your comments, thanks.

HiTech

One way to know if it is the vacuum booster is to hold your foot on the brake and if the pedal comes back up and you can't push it back down the the booster is shot. The brakes worked, you just couldn't hold your foot on the pedal very long before it came back up and you had to let off on it and push it again. The first time it happened i didn't know what was going on. lol Never had a brake pedal come back up at like. Once the pedal came up you lost the brakes and had to reapply them.  That happened on my Ford F-500. Had to replace the booster which was located behind the seat. I found by buying it through a Ford Dealer it was far cheaper than buying it at a parts store such as Napa.

BOBWOOD

Thanks for the help guys...I pretty much understand the system now I think...it is only boosted by engine vacuum pressure and not hydraulically from the power steering pump...I noticed some leakage from the ps rear...perhaps the fellow I purchased the truck from had things mixed up (he never actually used it himself)...I did open the bleeders at the master and booster cylinders and fluid shot out...no air...I have both front wheels off right now and will grease the bearings, clean the shoes and see about getting new slave cylinders...there are no adjusters on the front assemblies...just the 2 cylinders...thankfully it seems as though someone actually maintained this truck in the past so lots of the parts are in good condition...BOB

John Woodworth

BOBWOOD; I stand corrected, your method is correct. Double checked with chilton truck manual to be sure.
Two Garret 21 skidders, Garret 10 skidder, 580 Case Backhoe, Mobile Dimension sawmill, 066, 046 mag, 044, 036mag, 034, 056 mag, 075, 026, lewis winch

two-legged-sawmill

Hello BobWood, Lots of good pointers, Check the vacuum
going to your booster, on old vehicales sometimes have
leaks thru loose connections??? A gauge helps, good luck!
Dan
"There are no secrets to success. It is the results of preperation, hard work, and learning from failures"

Thank You Sponsors!