'76 GMC 6000 Brake help please

[C Carl]

In the decade or so I have owned this tough $600 antique truck, I have only driven it about 200 miles. It has a 350, and Allison transmission. A huge 4000# liftgate, and it is time to work it. All I have done is oil change, and trans filter and new ATF. Sat for a year or two, and when I fired it up and took it out for a test drive, there was no boost to the brakes. With about 3000# load, at 10 mph down a slight grade, I could stop it gradually with full 2 leg pressure. Brake pedal very firm with no bleed down whatsoever. Both front drums slightly warm after 2 or 3 stops.

 

Inspection shows no signs of leaking anywhere, but I can not pull the rear duals and drums out here. I did not do a temperature touch test on the rears. I doubt I could have felt any warming on the rears anyway. Brake fluid quite low,  but nothing normal brake wear would not account for. My feeling was that I needed a new booster. Since the system will be opened up, gonna put a new master cylinder on it anyway, and do an alcohol flush/new brake fluid, and bleed it.

 

Crawling under, the booster has been replaced before I picked it up out of University of Washington surplus. The cylinder on the firewall looks to be prehistoric.

 

Question is : Is there any possibility that just picking the low hanging fruit on the firewall, that this could be the problem  ? My intuition says no, but I have never worked on a system like this. Obviously, going to check all vacuum lines and connections. Would there be a check valve anywhere ? Any other tips such as bleed sequence (notice the bleed valve on the cylinder on the booster). 

 

Thanks  for any help in getting this rig to properly stop. Time and money scarce in these parts these days.     -    Carl 

 

 

 

[Bloo]

1 hour ago, C Carl said:

My intuition says no,

 

Looks like a simple remote booster. IMO if you have a pedal. it ain't the master cylinder. I wouldn't be in a mad rush to replace it, but on the other hand, it might not survive bleeding.

 

How sure are you that the booster has vacuum on it?

 

[C Carl]

I don't know if there is vacuum yet. Of course that should be verified. I have a guy helping me, because I cannot do much wrenching any more. He is inclined to replace the master cylinder, (which he has already bought), flush, and replace the brake fluid. Myself, I think that is pretty much a waste of time. The pedal is firm, and needs no pumping at all. The entire vacuum side should be the first step. Next a new booster. I need to get this wrapped up by Wednesday, and go back over the mountains Thursday. I am at near panic stage, and don't know how long it will take to get this booster to Ellensburg. Originally I had an extra week. But my friend had a family emergency, and now I don't have the luxury of the superfluous, extra, out of sequence step any more. If it absolutely can not be the master cylinder which you and I believe to be the case, then the fairly circuitous vacuum maze must be checked. Any tips there ? But I do feel that if the booster is bad, then might as well replace the old master, as it is cheap, and easy.

 

Does everyone agree with this analysis ? Anyone to the contrary ? I just do not have anymore time to lose. Is there any way in the world that the master cylinder itself could cause the problem ?    Thanks,  -   Carl 

[Alfre]

the brake cylinder on the buster is probably stuck.

Al 

[61polara]

Carl,

I think you have a Hydro-Vac system.  I don't completely understand them, but check your power steering fluid level.  I have a '75 Chevy 3500 with hydro-vac and if the PS fluid gets low I first get a pulsating brake pedal and if even lower no power assist.

 

[plymouthcranbrook]

GM began using the Hydro-Boost system many years ago(30-40). All Astro’s and many trucks use it. I did a quick search and found a lot of information about it available I would add links but on the I-Pad it is a pain

[Frank DuVal]

It is not Hydro-boost, you can clearly see the vacuum chamber on the booster. The firewall mounted master cylinder just supplies fluid to the booster's slave cylinder, which in turn pushes another master cylinder with vacuum assist movement. See second picture.

 

A hydro-boost (all the ones I have seen) are all hydraulic systems. no vacuum chamber, since it does not use vacuum at all.

 

My suspect is simple DOT 3 corrosion sticking pistons in the wheel cylinders, or worse, as Al says, the booster master cylinder is stuck from DOT 3 corrosion.

 

DOT 3 corrosion gives very stiff pedal, as there is hardly any movement of the hydraulic system due to stuck pistons.

[C Carl]

No, it is not hydro boost. My '76 Eldo has it, but the truck does not.

 

Frank, what you and Al say makes sense. It will probably be necessary to replace every cylinder in the system. Due to the size and weight of the wheels, drums, and the two ton truck itself, it is beyond my capacity at this time. In any case, I do realize that little used hydraulic brakes really should be completely gone through if they develop any problems. Short cuts in brake maintenance are not advisable.

 

Thank you all for your help. I think I need to come up with a plan B for the near future. Glad my two oldest vehicles have mechanical brakes !    -   Carl 

[Bloo]

Hydroboost and Hydravac are 2 different things, but often get confused.

 

Hydroboost gets its boost from the power steering pump hydraulically.

 

Hydravac is a remote mounted brake booster that goes back to at least the 40s and maybe the 30s. It is powered by vacuum. The one in the pictures might or might not be a Hydravac, but if it isn't, it is still the same sort of system, a remote mounted booster powered by vacuum. Similar things are used, for instance, on pre-1970 Alfa Romeos, 1971-72 Honda AN600/AZ600 cars, and probably a bunch of others. Not all are called Hydravac, but they all do about the same thing.

 

I cannot imagine an alternate universe where changing the master cylinder is going to fix this. It is good that you have one handy though. It might not live through the bleeding process.

 

I suspect either the lack of vacuum, or the booster. Yes, as Frank says, the cylinders might just be stuck.

 

[wayne sheldon]

Have a friend help. Jack up and safely block up all four wheels (it isn't a three axle is it?). Use engine to turn rear wheels (low or mid gear would be fine, doesn't need to be fast), and press brake pedal. Make sure both rear wheels brake evenly, and quickly. Then (engine off is fine), have friend spin front wheels one at a time. Make sure both of those brake quickly with moderate pressure. This should tell you if the wheel cylinders are functioning. It is also not uncommon for the mechanical mountings of brake shoes to rust and prevent the shoes from extending and doing their job. Either thing could prevent one or more shoes on one or more wheels to not function.

Another thought, I am "assuming it doesn't have disc brakes that early? Either way the same test works to test brake function and determine which if any wheels are not working. Disc brake calipers also can rust while sitting and fail to operate. I had that one happen myself.

Good luck! And drive carefully.

[jpage]

My son and I run a '75 C65 dump often in our construction business and the brake pedal is hard but it will still slide the wheels if you press hard enough. These truck are heavy and hard to work on without the proper tool and equipment. I would suggest finding a good hvy. truck mechanic who has experience with these vehicles to check it out. Saves alot of messing around and there's the sfety issue, your's and the others on the road. Hope all works out. I'd be interested to know what you find out.

[joe_padavano]

This truck has a hydro-vac system. Do not confuse this with hydroboost, that uses the PS pump to provide the boost. Hydro-vac systems have been around since at least the early 1950s and possibly even before that. Hydro-vac is a remote vacuum booster with a separate master cylinder. The firewall-mounted brake cylinder only provides pressure to the master cylinder on the hydro-vac. The master on the hydro-vac actually operates the brakes.

[C Carl]

Bloo, Wayne, jpage, and Joe : Thanks to you guys too for your continuing help.

 

Can someone please tell  me what the little white plastic device shown here is ? You see it to the left of the M.C., and a close up of top and side views. This is located at the very top of the vacuum system. Up from the port on the rear of the intake manifold, then down to the vacuum booster.

 

No, the truck has drums all the way around. Single rear axle, two wheel cylinders per each rear brake side. The way to test for function is to drive it up to 8-10 mph dnd stop the truck a dozen times or so. Then feel the drum temperatures. Left to right on each end should be warm and equal. This truck is way too heavy for my equipment. 

 

Assumptions, rather than experimental tests and a proper diagnostic trouble shooting sequence, are causing problems around here. In my experience, nothing wastes more time, and causes more problems then quick assumptions, rather than thinking things through to get answers. Oh man am I having an impossible time trying to learn up my young helper(s). They ASSUMED that the firewall mounted main cylinder was bad because it is very rusty on the outside. Well, I popped the top off it, and the reservoir is totally clean, and no rust. They also prematurely opened up the brake lines at all corners, before we could test anything. They seem unable to logically and unemotionally discuss the situation without delicate, youthful egos getting bruised. Their solution, after making a couple of time consuming mistakes, is to pick up their marbles and go home. But they are young yet. I think I can remember when I was a lesser mature kid their age. Yes, 58 years old seems young to me these days. How do you deal with a situation like this ?    

 

But what is that white plastic device anyway ?   -   Carl 

 

 

[Bloo]

Carl, the cylinder I sent out and had sleeved LAST YEAR (or was it 2 years?), and looked new at the time, looks like yours now....

 

That white thing must be a vacuum check valve.

Edited July 15, 2019 by Bloo (see edit history)

[C Carl]

Well, yeah, now he says he is going to just come up and hook up the right rear line he took off to "drain" the system, bleed it and walk away. Can a system like this be bled properly without a FUNCTIONING BOOSTER  ?   I shouldda stopped him dead in his tracks when he decided to replace a working,and cylinder. There are plenty of people out there who will not listen to anything. Sure makes life hard.

[Frank DuVal]

I would think the system can be bled without the booster working (as in boosting, it has to work hydraulically, or no, it will not bleed). But, I never tried to do one, so this is a SWAG.

 

So, how does brake fluid get to the booster? I see the label that says to check fluid every 30 days, so that one reservoir connects to both "masters*"?

 

 

* the one on the firewall that the pedal pushes, and the other one on the booster that vacuum/hydraulic slave pushes on.