Pls help brakes lock at bottom of pedal travel

[BP68]

Hey all.
I am having a brake issue. I have a 68 C10 with a 72 factory air cab, using booster and bracket off 72 truck. Master cylinder was new from CPP for a 70 model with power disc/drum. Brakes feel good as you press pedal. As pedal reaches near bottom of travel there is almost a faint sense of bottom, but if you press pedal till it reaches full bottom (maybe just under last 1/4 of movement, brakes lock up.

So a light foot brakes fell great, but as you come to full stop, if pedal is fully pressed till it stops moving there is lock up. Any ideas? It is as if brake pedal bottomed out just under a 1/4" less everything would be great.

Pls help

[gcburdic]

ttt

[red69ss]

do all four wheels lock up?

[BP68]

I need to double check, but pretty sure it is the rears only

[cdowns]

for brakes lockin up i was taught to pump the brakes rather then a full pressure panic stop and it will result in shorter braking distance

[special-K]

If it's not all four corners you have an issue in the corner or front/back. Front/back could be proportioning valve but could be a number of other things causing uneven pressure. The discs should do most of the braking. It could be you only have rear brakes. If it's all four I'd almost have to look at it to help. It's hard to know by description. It's normal for brakes to lock up at full hard braking,so I assume you mean it's at any speed while fully depressing pedal.

[68gmsee]

I agree with Special-K. A lot of things can cause what you are describing. First is to determine which brakes are locking up as mentioned above.

I'd try an isolated large parking lot and after the truck stops, go out and look to see which ones left the mark. (hopefully they won't mind you testing your brakes.)

Included in above possibilities are defective brake booster, proportioning valve, glazed or contaminated brake pads/shoes (my problem).

Here's what would happen on my truck. Similar to yours, on a slow stop there was no problem. On hard stop the rear brakes would lock up. New shoes and turning the drums fixed the problem. Brake shoes were contamnated/glazed and would grab as soon as pressure was applied. Since the rear of the truck is lighter the rear brakes would lock up quicker.

[jablack]

I've had the same thing happen that 68gmsee is talking about. I've also seen where the shoes were put on backwards, or both rear shoes put on one side. You have to look at your shoes because there is a front shoe and rear shoe for the rear brakes, might be the same for the front brakes but I've never kept front drums to know. Also check on the adjustment.

[BP68]

thanks for all the replies.

The best way to describe it is that if you lightly apply the pedal everything feels great,
it feels like the pedal is just at bottom of travel. If you slowly push that lat 1/4 inch it will throw you to the windshield at any speed, even on a cost to stop. That last 1/4 inch acts like you suddenly stood on the brake pedal with both feet, regardless of speed.

[68gmsee]

Originally Posted by BP68

thanks for all the replies.

The best way to describe it is that if you lightly apply the pedal everything feels great,
it feels like the pedal is just at bottom of travel. If you slowly push that lat 1/4 inch it will throw you to the windshield at any speed, even on a cost to stop. That last 1/4 inch acts like you suddenly stood on the brake pedal with both feet, regardless of speed.

Probably defective booster then.

[66nova383]

I know its a old thread but did you ever figure out the problem with your brakes locking up? I have the same issue.
every item in the brake system is new except the booster the last 1/4 inch of pedal travel will put you through the windshield.

[RodnRudy]

I had this problem on my last truck with power disc/drum. I plumbed the MC incorrectly, the resevervor closest to the MC mounting flange is for the rear brakes and I had it plumbed to the front. I had to ease the brakes on or it would lock up the rear, you might want to check that. I replumbed it the correct way and it solved the problem.

[special-K]

Could it be the wrong brake pedal push rod?