Brake Pressure Loss

[72BlckButy]

My FIL has a '69 SWB C/10 with a fairly well built 350 running a 292 cam, and it's running power drum brakes. Now for the problem...

When driving the truck it seems like the brake pedal and brakes don't have the grip they should give it's a power drum setup. Now I know drum brakes (even with a booster) are not like disc brakes but when I first purchased my '69 it would lock'em up without a booster.

Things we've tried:

Vac Canister - No effect
Replaced Power Booster - No effect

He recently added a 700r4 and had the shop check everything to ensure it was properly hooked up by the PO; everything looks right it just won't stop like it should.

We would like to determine the culprit without the need to upgrade to disc brakes.

Does anyone have any ideas?

Thanks in advance guys! (Pics of the truck to come)

[Lee H]

Will it lock up if pressed hard? Are the shoes and or drums glazed over? How much vacuum is the motor making at idle? I'll assume the shoes are properly adjusted.
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[fastwillie 696969]

pictures of shoes and hardware please

[chevytruckluver]

The title of the post says "pressure loss" yet you changed out the vac canister and booster are you feeling like its a hydraulic issue now?
when you changed the booster did you notice any brake fluid inside the old booster where the push rod is?
have you looked at the wheel cylinders?
have you tried bleeding the brake fluid?
also as asked above be sure the shoes are not "glazed"
good luck

[68gmsee]

Usually when you have a sudden loss of hydraulic pressure you check the brake lines, brake cylinders and master cylinder for leaks first. If there's no leaks and it's something that just happened all of a sudden, the master cylinder should be the next suspect. There are some seals inside that leak and allow fluid to be bypassed and not go to the brake cylinders.

[Sammy]

I personally would want to pull both rear drums and visually check some things. The glazing, evidence of the wheel cyl are not leaking and the axle seals are not leaking grease on the shoes. ( when a truck sits a while those axle seals glaze the shoes in a hurry) Insure the adjustment on the shoes is right upon reinstallation. Then check the front wheel cylinders for signs of leakage on the shoes and readjust the front shoes. Re bleed the system. If you find no evidence of leakage and braking doesn't improve to what your expecting, I would be looking more closely at the master cylinder provided you have good vac pressure into the booster.

[MagmaJct]

I've never worked on a brake booster, however I have experienced hydraulic pressure loss in a different vehicle. The master cylinder was bad and the fluid would bypass the rings. But this didn't make the vehicle hard to stop, it stopped well, but when sitting at a light or something, with my foot firmly on the pedal, the pedal would slowly sink to the floor. Pressure loss would do that.

But, when I first got my truck, the first vehicle I owned with an automatic transmission, I didn't realize the idle was set too high. This made it a bear to stop. I checked everything in the brakes until I checked the idle speed.

The torque converter is designed to transmit very little torque at low RPM (like idle). But as RPM increases, torque does so too, but at an exponential rate. Once I set the idle speed correctly, the brakes worked like a dream. So basically, I was fighting the engine every time I wanted to stop. What's worse for me, not having power brakes, I had to put a lot of effort into the brake pedal to keep the engine from moving the truck.

I almost rear ended someone in a drive thru because I didn't realized I relaxed my leg a bit!

[72BlckButy]

Sorry guys... been away for a few days. I will print out your recommendations and we'll start looking at the truck. It's been crazy hot here the last few weeks so it might be a bit before I can get my FIL out.

Thanks for all your recommendations!