The Stovebolt Site Forums The Shop Area Engine Shop I'm not getting the brakes

I recently got my 46 coe running and have only taken it on short drives around the property. My problem is by the time I get back to the garage the brakes are almost locked up on all four wheels. It rolls fine from the start. I first thought that it was because I had not installed the vacuum line to the booster, but have since done that and it makes no differense. I also pulled the front wheels to make sure there wasn't a problem with the cylinders or the adjustment but everything is ok. It seems that after it sits for a while or the next day the wheels spin free. Could air in the booster be the problem? The pedal is ok and the truck seems to stop decent. Any help would be appreciated. Thanks, Bill.

Could be the bleeder hole in the master cylinder is plugged. Should be 2 holes in bottom of master reservoir, a bigger one and a very small one. If the bleeder hole gets plugged you're basically pumping up the brakes each time you hit the pedal and it can't bleed back fast enough to fully release and the wheels will eventually lock up. After it sits awhile they bleed off and the wheels free up.

I agree on the valve in the master cylinder. Another possibility is that the return springs for the brake shoes are not installed correctly.

or yet another options is that the brake hoses (flexible rubber) are bad and acting like check valves. Allowing fluid to go to the w/c and not return to the master cylinder.

Let us know what you find.

One more thing you might check is make sure you have a little bit of free travel between the master cyl. and the brake pedal. If you don't have the brake pedal keeps the fluid from returning to the master cyl. Also make sure the master cyl. is returning all the way back.

Thanks for all the replys, I'm still at work but when I get home I hope to check some of these things. Most everything is new, wheel cylinders, all the lines, hoses, the master cyl., shoes and springs. I guess that that doesn't mean something could of gotten in there though. I'm pretty sure the springs are on right.
Thanks again, I will let you know what I find. Thanks again. Bill.

There is one more thing. You said the master cyl. is new. If this has a brake booster[hydrovac] that is mounted on the frame [not part of the master cyl. assy.] then when they made the master cyl. they propably put in the residual check valve. This valve will keep a small amount of pressure in the brake line going to the brake booster. That would be why it is not letting all the fluid return from the brakes. You can check this by removing the master cyl. and dissasembling it. Look for this residual check valve on the end of the return spring. If it is there, just remove it and put it back together.

racecarldad, I did check the pedal and there wassn't any free travel. I adjusted it and will have to try it out tomorrow. The truck does have a rebuilt booster and I don't remember having to specify if the master cylinder was being used with a booster so what you say makes sense. You think my new master cylinder has this check valve? I don't see a check valve in the manual. How difficult is it to remove it as it will be my first master cylinder surgery.

Lack of a little free play in the pedal may have been your problem. If 'twere me, I'd want to know for sure about the residual valve. You should be able to find out without removing the cylinder. Disconnect the brake line out of the cylinder, and remove the banjo bolt and fitting if your cylinder has them. Find a piece of baling wire or similar probe and see how far into the outlet port it will go. If there is a residual valve, it will only go in about 1-1/2". If there is no valve, it will go in maybe 3-1/2" or so.

If there is a valve in the cylinder, you *might* be able to remove it without taking the cylinder out. I don't know how much room you have to work. Most of these cylinders have a hexagonal headnut--the big threaded outlet adapter at the back of the cylinder. If you have a hex headnut and a wrench or socket to fit it (probably 1-5/8") you can remove the headnut. The spring and residual valve will follow it. Pop the valve (a little derby-hat-shaped metal gadget) out of the spring, remove the rubber gasket from the inside end of the headnut and put the spring and headnut back in. If your cylinder has a headnut with a serrated rim, you will want to remove the cylinder from the truck and take it apart through the front (pushrod) end. It would be physically possible to do it in the truck, I suppose, but feeding the piston in against the flare of the rubber cup and getting the spring clip into the groove while holding the piston against the return spring is difficult enough in a vise at the bench. I wouldn't try it under the truck.

If you're lucky you will be able to pump the air out of the cylinder without having to bleed the sytem.

Joe

Joe, I will pull the line off tomorrow and will see if there is a check valve. That will be difficult enough in my truck since it is a coe. Most likely if there is a residual valve I will have to pull the mc. Not looking forward to that. Now that the truck is back together it is a lot harder to work on. Thanks for your help. Bill

The GMC shop manual I have says all models are equipped with residual check valve, except series S400-24.

My truck had a residual valve in it; so when rebuilding it I replaced it. Was this wrong?

Maybe that's why I have had so much trouble bleeding my brake system.

Oh, I have a Hydrovac with Two (2) bleeder screws, one on the slave cylinder and one on the control valve.

Thanks everybody for all your help with my brake problem. I finaly got some time yesterday afternoon to spend on my truck. I kind of feel a little stupid for not adjusting the free play properly before. Thats what it was, and the brakes work great. Bill