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my 51 gmc 720 has a double diaphragm booster on it which im not so sure that its working properly and dont know how to tell if it is or only partially working. the info on it says "midland 300308"

yesterday when i started the truck i had my foot on the brake pedal. when it started the pedal moved slightly to the floor which tells me something is happening with it. when i was driving it to a garage down the street from where its parked to pull the wheels and inspect the brake system parts and it didnt seem to be working.i had no strong braking action or any power assist to do any braking even with a full hard pedal it was like there was no response from the brakes at all except for a slight drag.

i did notice while in reverse i had srong brakes?? if i this much brake going foward i wouldnt be posting this.

i know this is not the original booster. i can see the marks on the frame where it was mounted previously.

how do singe and double diaphragm differ in operation? :confused:

Hy 51ashton, It sounds to me like you already inadvertantly performed one of the checks for booster function. When you held your foot on the brake pedal and started the engine and it went closer to the floor that indicated that the booster was functioning. It is a hard concept for some people to get their head around, but brake linnings can become glazed, that is they develope an extremely hard surface being used in the same direction all the time. I think that is what your dealing with now. To deal with this you can try a couple of things, first try a few hard stops in reverse, say five or so from 10 m.p.h. Now see if your forward stopping power improves, if it does you're on the right track, if that doesn't help remove the drums and scuff up your linnings with a fairly coarse emmery cloth then blow off the linnings and reinstall the drums and test your brakes. If neither of these tricks help I'm afraid it's reline time, hope this helps.

the linings didnt appear glazed, actually everything in there (under the drums) looked fairly new and in perfect condition compared to what i thought i was going to find.

Hy 51 ashton, looks can be decieving, you said in your post your brakes were better in reverse, that's your major clue to the glazed linings, the drums also get polished as well ,they're not supposed to look like your chrome bumper on the braking surface, hope this helps.

3b, can you scuff glazed linings? I have always wondered about that. Like hitting them with 60 grit to pull off the glaze?

Hy arnswine, All you want to do is break the surface so you can make the linings surface work against the drum rather than just continue to polish the drums. Probably the best thing to do would be to take a skim cut out of the drums as well, but that is not always practical especially with the larger drums.

3b, what is the major cause of glazing?

Hy 51 ashton, that's a very good question. Some people say it's major cause is too many stops in one direction, causing the lining to compact and develope a hard surface, others theorize that the metal dust from the brake drums is driven into the pores of the lining over and over again causing the lining surface to harden, both these theories are based on the theory that the lining cannot clean itself unless you occasionally apply pressure from the heal to the toe rather than from toe to heal all the time. These theories seem to be born out back the fact that city buses seem to develop glazed brakes more often than other heavy duty vehicles. Incidently how did you make out with your brake function problems?

3b, i havent yet. im waiting for title transfer, antique tags. i have insurance and should have new title this week. the truck is still sitting where is has all of its life. when all the paper work is completed ill relocate it to my place.

then start the brake process starting with the master cyl. prior to that ill try the reverse technique to see if anything improves or not then decide how to procede.

i have to keep the priority on my engine for the time being, things are starting to come together on it very quickly.

Has this truck been sitting for some time?

It almost sounds like some of the wheel cylinder pistons might be stuck (and not applying that shoe)...

Also, make sure the shoes are properly adjusted...

frank, the truck was taken out of active service in 1999. the truck as i know of it was driven on occasion and stored in the station. i was told the reason they stoped driving it was because the brakes went out and they claim it is the booster that is the cause. how they came to that conclusion i dont know. when i got my 51 350 i went over everything a couple of times before i was satified and knew the system inside and out ill be doing the same with this one.

the first thing i did 5 min. after i gave them the check for it was to pop the top off of the M/C and it was bone dry. i wasnt able to determine at that time where all the fluid went.
i filled the system and bled the system making two rounds of bleeding everything. the H/V has no bleeders on it unlike my 51 350. i was able to get enough brake pedal to feel confident that i could road test it safely. after the drive i found the M/C was dripping.

last weekend i pulled the wheels for casting info and a general inspection. i could hear all the drums making contact before pulling them. everything looked good for the exception of one rear W/C had a slight visible weep.

as for checking the W/C for operation ill have to do that when i get it to its new home.(its still sitting in the fire station)

any suggestions on checking the W/C for operation other than trying to spin the wheel with the brake applied?

I would suggest you pull each wheel and drum and do a visual inspection of the W/C pistons. I say this because if one side of the cylinder is spreading and the opposite side is not, you will get enough pressure from the one side to stop the wheel from turning.

You will need a sledge hammer and a propane torch to remove the drums unless they have been removed recently. Remove the wheel then remove the drum retaining screws from the front side of the drum, your torch may come into play here because those retaining screws have a tendancy to rust tight to the drum. I use an impact socket driver for rusted screws,one of those you hit with a hammer,that seems to work well for me. A word of caution, dont overheat anything and try to keep the heat directly toward the screw or part you are trying to remove.Loosen all adjusters so you can spin the wheel freely with zero drag.Heat the front of the drum where it meets the axle(between the lug studs and around them)and tap on the back lip of the drum with the sledge all the while keeping the heat on the front of the drum.When the drum breaks free it will most likely come right off if you have the adjustment backed off far enough.

That double diaphram booster has no bleeder(at least I don't beleave it does)but there should be a fluid fill port with a plug and you will need to put a pint of fluid in that fill hole as part of your bleeding procedure.

Best way I have found to do that is by opening the bleeder on the passenger rear wheel and have an assistant pump as you add fluid to the booster.You know the drill,add fluid,open bleeder,pump,close bleeder,pump and release pedal,add fluid and so forth until you get that booster to accept a pint or so fluid.

My big truck has no bleeder at the booster but has two on each of the rear wheels,one for each of the two wheel cylinders per side.

The shop manual just says add one pint of fluid to the booster but gives no instruction for getting the air out so it will accept the fluid.

I would check the connections on that booster because you said it had been replaced. There is a possibilty that the lines were connected in the wrong place.The quick check for this is to have someone pump the brakes and hold pressure while you crack open what should be the line going to the rear of the truck and see if you lose pressure at the brake pedal,if you lose pressure then all is okay with the connections.

Be sure and check the adjustment on the brake linings by getting the wheel off the ground enough so that you can turn it by hand. Adjust the brakes tight until you cant turn them by hand at all,then back off that adjustment until you can turn but still have some drag on the wheel..Remember, that truck weighs a bunch and what you normally call drag means nothing when that truck is rolling, so you want more drag than normal. If you have a brake adjusting tool the stroke toward the axle loosens the adjusment and away from the axle tightens it.

To futher help diagnose the problem I would need to know if you have AOHB(air over hydrolic brakes) Full Air or Vacuum Assist.
Good luck and I hope I have helped a little.Hobert