1969 442 stock brake setup, disc front, drums back. Converted to slicone brake fluid several years ago.
Recently, a firm tap (not even a hard stop) on the brakes can make them lock up. Not sure if it's the front or back that are locking up but think it's the back. A softer pedal will not lock up.
There is a metering valve under the master cylinder and a distribution block underneath in back of the driver wheel well...all stock...and stock disc and drum brakes, lines, etc.

Brake booster is original and has never been opened up...n either have the metering valve or distribution block.
Brake warning light comes on at start and then goes off (correctly).

Any ideas?
Thanks

 

Read this thread on rebuilding brake distribution block. Not sure if this is the culprit though as my warning light is not coming on except at start as it's supposed to.

https://classicoldsmobile.com/forums...estion-164804/

 

Have you checked for fluid leaks?

 

Yes, no leaks detected front or back brakes, distribution block, or metering valve/master cylinder.
No detectable fluid loss in master cylinder either.
I should have mentioned that.

 

An oil on the shoes from axle seals?

 

Are they
#1 Locking up ? or
#2 Grabbing when you "tap it hard" ?

 

They are locking up in that the tires screech/temporary loss of traction until I let off (very startling the first time it happened as I was not expecting it).

 

Have you bled all four corners?

 

That is where I am going to start including thorough inspection of brakes/pull the wheels, etc., but no leaks on the floor or on the inner wheels.
This came out of the blue and it is a lock up. Something definitely changed.
Just trying to come up with a plan to check the most likely culprits...clogged up cylinders/lines (witch i doubt because I went through them in past 5 years), distribution block, metering valve, vacuum booster (all originial)???
Start with the basics though.

 

Look closely at the rear wheel backing plates on the bottom for any sign of oil or dust accumulation.

 

Does the car sit for a long period of time ? You may want to "test" the brakes for "grabbing" a few times after you pull onto the street. Perhaps rust formed inside and that is causing them to grab. A few slow speed stops will help before you take off.
If you feel them get less sensitive, then you solved the problem. Your rear drums are more likely to grab than the front discs.

 

Old brake hoses can internally deteriorate and cause what you describe.

 

Thanks. That's first on the list. The car does sit a lot ~ 7 months and then maybe gets 500-1000 miles in the summer/fall months.
Last summer it was driven very little...so...maybe that's the explanation.

 

Father time can do it but I do not see sporadic use having any effect. If the brake hoses are original, they need to be changed out anyway. I experienced this myself once with 40 year old original hoses. Just a light touch on the brake pedal would cause the rear tires to lock up.

 

Problem solved (I hope)...took OLDSter Ralph's advice...took it out for a drive and worked the brakes pretty good. Locked up once first time then no more...they seem to have quit grabbing...at least after they got warmed up.

I'll try again fresh/cold tomorow.

Thanks Ralph and thanks to others for responses.
Andy

 

You don't have to do it every time you drive it, just when it has sat for a longer period of time. A few (2-3) low speed stops can be helpful to test the brakes and know they're operating properly, any time.

 

Again. Have you bled all four corners?