Back end comes around under panic braking.

[jtinpdx]

Hi All,
I can't keep my truck straight under hard braking. The backend comes around really aggressively. It's a 71 with power brakes in the front and drums in the back. Longbed. Posi-traction. It also has a trailer assist brake installed.

The brakes have been inspected and pads replaces recently. New master cylinder also.

Any ideas about what to look for?

Thanks,
Jeff

[GRX]

Did you set the pin on the combination valve?

[RustyBucket]

My '71 GMC did the same thing, rear end always came around to the right. R.R brake adjuster was was stuck. Freed it up, adjusted drums and it'll lock all four and stop pretty straight now.

[jeffahart]

The rear brakes are not coming on. It's the rear brakes that keep you straight when you brake. So I would check the metering block. I am assuming you have disc front when you say power front. So you would need a metering block. Metering block holds pressure off the discs a bit until a certain pressure. But the 71 might have a combo valve. Anyhow I would verify correct parts and setup. Someone could have changed some brake components.

[GASoline71]

Are the rear drums locking up? Could have gear oil leaking past the outer axle seals and if it gets into the drums it makes the shoes stick to the drums when the brakes are applied.

Gary

[Chknlyps2]

Find a dirt road or parking lot that is level, go about 10 mph or so and hit the brakes hard enough to skid to a stop and park right there. (Looseness of the dirt/ traction will dictate speed.) You want to get out and see which front brake locks up first, if you went to fast your back tires will pass up where your fronts engaged and erase the starting point of the fronts. If both start to skid close to the same place then go a little faster so you can get the rears to lock up and see if they both start skidding at the same place. You can rule out or confirm a non working wheel or whole axle this way.

[old Chevy guy]

I'm no expert and I don't play one on tv either but I remember in my younger days we used to jerk up on the parking brakes in our cars and spin them around 180 degrees. I'd be thinking rears are locking and fronts not so much. Try that dirt road skid suggestion that will tell you what's going on.

[Rich69shortfleet]

Originally Posted by GRX

Did you set the pin on the combination valve?

How do you do this?

[jtinpdx]

Thanks for all of the suggestions, guys. I have some research to do. I'll do the skid tests and let you know what I find.

As for the prop valve, yes I think it's the right part and has been set. The dash light does not go on, and I know the dash light works. The reason I have a new master cylinder is that the light was always on. The master cylinder was leaking out of the one of the pistons, not sure which. That was replaced, and I assume the prop valve sender was reset, since now the dummy light isn't on.

But worth checking.

But that's why I mentioned the trailer brake assist. There's a brake line splice off of the rear brake line for the trailer assist. I wonder if that isn't causing some issues. I plan to remove it. I should do that now and see if it helps.

[kwmech]

Air still in the system after the MC swap?

[Grumpy old man]

What ? You mean your almost 50 year old truck didn't come with ABS . Heaven forbid !

[ga-stepside]

Originally Posted by Grumpy old man

What ? You mean your almost 50 year old truck didn't come with ABS . Heaven forbid !

LOL. In 1993 I bought my first truck with ABS and two weeks later I had a major panic stop and was amazed to see ABS actually works!

[Grumpy old man]

Originally Posted by ga-stepside

LOL. In 1993 I bought my first truck with ABS and two weeks later I had a major panic stop and was amazed to see ABS actually works!

Try the same truck in ice and see how that works for ya ?

[ga-stepside]

Originally Posted by Grumpy old man

Try the same truck in ice and see how that works for ya ?

Nothing works on ice but experience. Been in Georgia for 27 years and have probably lost my snow and ice driving talents!