Just when I thought my brakes were good to go
 

People, I only have 1 day I'm weekends to do any maintenance type stuff due to traveling back and forth to the bay. After 1 month I finally get to drive my truck. Now at odd times my rear brakes are pulsating/grabbing, especially at sudden braking. What gives? To get you up to speed on my set up, about 6 months ago I swapped all rear drums to 3" shoes, larger cylinders and new drum springs, hardware, adjusters'etc. new booster about 4 months ago. I already bypassed the RWAL valve. Disconnected the abs module. Help

Could it be a rusty spot on a drum or two?

Since the rear drums are the same for both 3/4- and 1-ton trucks, 3", I'm assuming you only installed new shoes? If so, did you have the drums turned/machined back to true?

no I did not. If this is the root cause of the brake grabbing I had it coming to me then. I told myself at the time to get the drums machined but the machine shop was closing in ten minutes when I called and closed the next day so I just buttoned it all back up. Dang!!! Had a feeling this would come back to bite me!

last time that happened to me the reason was axle oil started leaking into the drum and fouling everything.

Thanks for the assisting suggestions,all. I'm curious if master cylinder faults would cause this. My raybestos MC is about 6 years old. Also what about improperly adjusted rear brakes? Seems this all started after I took the truck out to the back of a grocery strip mall plaza to back in reverse s few times to tighten up the rear shoes and stiffen up the brake pedal. If my pedal fades and stiffens up with an immediate pump on the brake pedal that means what again? Man I thought I conquered my brake issues when I did all the upgrades. Guess not

Had the same issue,
Turned the drums and installed new hardware while I was in there and the brakes were smooth,

So, I tackled this problem today, and rather than tear into things and throw parts at it, this is what I checked: 1. Level of brake fluid- still at 1/4" down from top of MC. 2. Signs of rear end gear oil dripping wet around the rims and tires which would soak the liner of the brake shoes, causing the grabbing. None shown. 3. Had my son pump pedal 6 times and hold while I bled all 4 bleeders, one at a time with my son pumping and holding for each bleeder. Went around the truck at 4 sets. Brake pedal stiffened up, so I did the reverse thing to adjust drum starwheels. Feels good so far. My question is: how many cycles do you go when bleeding brakes. Doing all 4(my RWAL is bypassed) constitutes 1 cycle. Tomorrow I will check the level of the gear oil in the rear to verify truly that it's not going down by leaking past the seals and soaking up my shoes.

I start at the wheel the farthest from the master cylinder and work the next closest wheel until there is no bubbles in the fluid.
My old Chevy liked to be bled one day and then the next to really firm it up.:conf:

You should inspect inside the drum anyway. I've had the seals leak but not enough to blow out all over the drum's exterior... just on the shoes.

No bubbles in the fluid are generally what I shoot for, using my peanut butter jar and clear tube gizmo I made. It captures the fluid, and prevents back sucking of air into the bleeder while bleeding. It's cured most of my issues, as I can tell just whether or not there is air coming out of the system during bleeding.

Almost every time I've encountered the issue with the brake pedal going soft it was because of air infiltration at the rear wheel cylinders. Either the bleeder wasn't tight enough, or I was opening the bleeder too far during bleeding ( not just the minimum needed to get the fluid out) again allowing air to enter, or the tube which entered the junction Y or the rear of the wheel cylinder wasn't tight / seated, allowing air to enter. Human error, I'd call it on my part.

On the rare occasion, the front bolt and copper washers of the front brake hoses didn't seal the first go around, but after snugging them up, that was fixed.

Pedal that works for a while, then goes soft is a sign of air infiltration into the system, most of which are on the rear sector of the truck. This has been my experience.

T- good stuff as always. Thnx, and I'll check all those things

Update: brakes so far working better and not grabbing after 4 cycles of bleeding. Now, though, my brake light keeps coming on. This is odd to me since I bypassed the RWAL valve and disconnected the abs modulator in the glovebox. I did keep the harness to the RWAL plugged in to avoid rusting out exposed ends of the connector. Would this cause the light coming on?

The piston in the balance valve isn't centered.
Some times when bleeding one end more than the other the piston that balances the front/ rear can get stuck. I have fixed mine by opening one end Front or rear. Just like bleeding.(besure the brake pedal in all the way down.} if that doesn't work do the other end. This has always worked for me.

Could my vac pump be failing?

More likely the balance valve piston stuck. or even the switch sticking.
If you have a vacuum pump problem you would have a stiff pedal. and have to push harder.

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Ours work the same way but the valve is inside the frame under the master cylinder.