The Stovebolt Site Forums The Parking Lot Big Bolts Looking for brake leak.

I lost my brake pressure the other day.... right to the floor with the pedal. Now just to inspect the rear brakes I have to pull the axle to get the drum off. Any tips before I get into it?

if you had a "catastrophic" fluid loss it should be easily visible someplace, but it may also be that you didn't know the fluid was very low until it got to the point there wasn't enuff to keep the pedal up - check the master for level, refill and pump for awhile and see what happens

look all over underneath, check for a blown rubber hose .... check the fronts first, pull the adjuster plugs in back and shine a light in to see if it looks gummy .... if it lost a lot of fluid via a back cylinder there should be evidence of it around the drum edges at least

Bill

Thanks for the quick response. I did a sight inspeciton around the backing plates, but will look for more eveidence. I have had this truck for about 6 months, but haven't really gotten into inspecting everything. I was going to start on a wiring harness this week, but this is more pressing. I did see that my passenger backing plate has a pie shaped piece broken off. I'll be searching for one of those.

No blown lines, and I didn't pull the drums since it is a major service in the 48s with the inboard brakes. I topped up the fluid in the master cylinder and pumped the air out. I have brakes again, but I must have a leak somewhere, even though it must be slow. I have to replace the backing plate on the rear passenger's side though. I am just getting to know this truck and I discovered a pie shaped piece missing from it. That can't be good.

Pull the vacuum hose off the booster & see if any fluid is in it. Fluid can leak in the booster & you won,t see it. Look where the rod goes in the master cyl. A very common leak source.

I'm driving a 49 ton and a half and I recently worked all the brakes over. Pulling the rear axle isn't nearly as scarry as it sounds. If the truck was setting around for quite a while (like mine) the master and wheel cylinders rusted up pretty bad and I was losing fluid everytime I hit the brake.

I have a 53 6400 and have had the axel out of it. There isn't really much to it. If I can do it, I'm sure almost anyone can. As to the brakes. I have had an awful time with mine, and I suspect most of the problem is in the booster. I can't find any evidence of leaking fluid anywhere, but I can bleed them and think I have everything ok, but gradually I will lose the pedal. I think the problem is in the booster. I don't think I am getting all the air out of it. I just recently went over them again, and this time I used a vaccuum pump at each wheel, and on the two bleeder valves on the booster. I could see a lot of air bubbles when I was working on it.

The shop manual calls for a pressure bleeder to be used on the master cylinder, forcing the fluid and air through the system. I don't have anything like that, and wouldn't know where to get one. It would probably cost more than I would want to pay even if I knew where to buy one. Allen

Bakerallen, have you tried any of those so called 'one-man' bleeders? I've seen some that look like they would work, kind of, but never tried on. I think the principle is that there is a check valve in a piece of tubing that runs into your transparent jar so you don't need to tighten the bleeder after every pump. That still leaves you with pumping the pedal and making sure that the master cylinder stays topped off.

I also read somewhere that expediency in manufacture sometimes means that the passenger's side rear isn't always the bleeder that is farthest from the master cylinder in terms of tubing length.

On a few occasions I've bled brake systems by bleeding the cylinder farthest from the master cylinder until I get steady clean fluid flow there, and then working back towards the master cylinder.

One thing I didn't think much about until I read the owner's manual for my 1990 BMW is that brake fluid absorbs water whenever it can, and the water eventually causes the lines, etc., to corrode, with predictable results. Part of the recommended annual service is that the brake lines be flushed completely.


Just a few thoughts, that I hope someone will find valuable.

Thanks for the suggestion: I have never seen the tubing with a check valve, but I did find bleeder valves for the wheel cylinders that have a spring loaded check valve in them. They seem to work well. I could not find them for the booster, because it takes a bigger valve. I think the problem is in the booster. I don't think I am getting the air out of it. If my pedal goes away this time, I am going to take it to an old shop that I think has a pressure bleeder, and see if he can make it work. If that fails, I am going to do away with the booster. I'm not going to haul anything heavy, and plain old brakes will be fine. Allen

Is your booster the type that mounts to the frame behind the passenger's side of the cab?

Just a few bits on that kind of booster. As I understand it, they have leather seals and are supposed to be 'lubricated' regularly. My truck sat for 12 years, so I'm going to do what Grigg did and see if my local NAPA has a rebuilt one. I don't know how long Chevrolet used this style of booster but they do seem to be available.

I'm kind of interested in finding out how well they work. I'm hoping that the new engine will still produce enough vacuum to run it without a separate pump.