1961 GMC 4000 Bleeding Brakes After Master Cyl. Replacement

[trav_rich]

I have a 1961 GMC 4000 with power brakes. I had the brake booster and master cylinder rebuilt. Directions and brake shop were adamant that the system could not be bled without a "power brake bleeder". They say it is not possible to bleed system due to this master cylinders complex design. The power brake bleeder was apparently a machine you could hood up to the system and put 50 lb pressure(maybe vacuem) and bleed system. I am told these machines are not around anymore and can not hardly be found or purchased. I was told not to hook vacuem to booster until bled or damage could occur. Any advise?

[64fleetside]

Check some older repair shops in your area-back in the day every shop had one, maybe someone still does.

[aerotruk63]

Went through the same thing with my 64 GMC 6000. Had mechanics tell me they could do it by other methods and failed.
My Dad made me one out of a 2 1/2" pipe about 8" long with two threaded caps at each end. The cap on the bottom gets a fitting to attach a hose to allow the brake fluid to go to a flat plate with a fitting to allow the fluid to continuously fill the master while you open the bleeders starting from the furthest point to the closest.
The top cap has an adapter for allowing about 5 psi of air pressure to pressurize the system.
The flat plate replaces the cover on the master. We installed an O-ring beneath the head of the original bolt that will clamp the flat plate to the master cylinder. The flat plate will need a gasket between it and the masters perimeter.
Fill the master first, then attach the Pressure Bleeder flat plate. Fill the Pressure Bleeder with brake fluid and install the top cap. We used a hanger to keep the Pressure Bleeder above the master.
Make sure all your bleeders are working before you start to bleed. Once your set up it will take about 15 minutes to bleed the system.
IF YOUR BLEEDING OUT A LOT OF BRAKE FLUID BECAUSE OF AIR, DON`T ALLOW THE MASTER TO RUN OUT OF OIL.
The original pressure pots held a quantity of brake fluid. They were the size of one of those beehive vacuum cleaners.
I use the adapter on my 63 C10 as well since the rectangular master is similar.

I`ve seen a plastic one on the internet somewhere.

[QBA]

did you "bench" bleed the the master cylinder before you intalled it on the truck?

in other words run a line fron the output of the m/c back into the resivior,fill it with brake fluid and push the plunger back and forth til no bubbles are seen coming up from the line going into the resivior

hope that makes sense.

[aerotruk63]

The master was already on the truck. The 5 psi oil stream took care of everything.

If you google Pressure Bleeder there`s several sources to buy one but you can make your own much cheaper.
My truck broke down a couple of hundred miles from home but we were lucky enough to find a retired mechanic that had one, he would not sell it though because he could bleed any car with it. As long as the adapters fit.

[trav_rich]

To help clarify:

1.Yes the shop that rebuilt the brake booster/master cylinder bench bled the master cylinder.

2.With the vacuem unhooked from the booster, I have attempted to bleed the brakes on the truck at every wheel cylinder. Brake fluid comes out at the wheels, and with little to no air. Yet the when you first press the brake pedal in it is very spongy and goes basically to the floor. If you keep pumping it starts to firm up then you can hold it and bleed a wheel cylinder.

3.Would the hand held vacuem brake bleeders sold for around $20 be of any assistance?

Thanks for all your replies.

[ABX131]

Is there a slack adjustment between the pedal and the M/C? May need to be adjusted to accomodate the extra play in the piston to pedal connection. When the pedal is pressed it may not be extending far enough into the M/C to create the pedal pressure needed.

[trav_rich]

Keep in mind this is a 1961 GMC 4000, I usually refer to it a 2 Ton. I think the brake pedal runs connects to the resevoir on the firewall, which I have never removed. The brake booster and what I usually call the master cylinder are under the cab and were rust removed, rebuilt, and reinstalled. Since the pedal and resevoir it attaches to were never removed I do not think they need adjustment between them. Before vacuem booster went bad the brakes worked good and pedal stopped in normal position. The pedal now goes to the floor even though I have tried to bleed everything. (Goes to the floor when you press first time, then firms up as you keep pumping.) I assume it is still air in the line or mastercylinder.

[j.kirn]

I have had the same exact problem you are having on my 64 c-60. i could not get the air out of the system, until i got a pressure bleeder. Once i did that no problem

[trav_rich]

I built the pressure bleeder, thanks to aerotruk63 instructions, and it work great blasting brake fluid to anybleeder i open. I have pushed fluid out at both bleeders on top of the master cylinder and all wheel cylinders. I still have no pedal, it goes to the floor when pressed on. I am gonna plug the line that exits the master cylinder and goes to wheels, bleed mc and try pedal again. Then i know problem is with mc and not at wheels/brakes. Is it possible the my rebuilt mc is defective? Any more truobleshooting suggestions, i am not used to having a master cyl. Resevoir on firewall with vac booster and mc under cab.

[trav_rich]

Finally solved the brake pedal going to the floor problem. It was not air in the lines. The problem was the check valve on the master cylinder was stuck open. The brake fluid was just going up the line to the resevior, instead of being pushed out to the wheel cylinders.