Weird Brake Problem
 

OK guys, here's the deal...

The Car: 1959 Chevrolet Impala

Front Brake calipers: '69-'72 GM A/F body (2-3/4" piston)

Rear brake cylinders: '59 Impala (1" bore)

Master-cylinder: GM 1-1/8 bore disc/drum

Combination valve: '69-'72 GM Intermediate

Power booster: Astrovan hydroboost

All new lines and flex hoses

The car stops great. It stops well with all four crying. But, if you press super-hard on the brakes while at a stop, you can make the brake light come on.

At first I thought it was a master-cylinder... replaced it. It's not.

Went ahead and replaced the combo-valve. That wasn't it.

I have bled the thing absolutely to death.

What am I missing?

Thanks!!!!!!!!

OK, just a WAG, but when you stopped hard, all the brake fluids in the m/c shoved forward, tripping the brake warning light?

Good thought, but it doesn't have a fluid level sensor. Also, it happens when you press very hard on the brakes and the car is not moving. :headscratch:

You might be flexing something enough to cause that combo-valve to trigger the light. You should be able to unplug the valve to verify that as the source. I would guess the only other source to trigger the light is the E-brake.

It's definitely the combo-valve triggering the light. Is it possible there is just enough flex in system? I'm tempted to put braided flex lines to see if that stops it... Argh.

You could be flexing calipers, drums, brake shoes or any number of things. That hydra-boost is probably boosting your brake line pressure far above any real world needs when you are pressing "super hard" on it. I'm not sure you really have a problem as you would most likely never put that kind of pressure on the brakes even in an emergency stop. You would most likely lock up the brakes first but you could test that in a safe area with no traffic/people and lots of run off area.

That's kind of what I am thinking... but I also keep thinking that the original Astro wouldn't trigger the light in such a situation.

If your brake warning light is connected to the pressure differential switch in your combination valve, the cause is an abnormal pressure difference between the front and rear brake lines. I have to agree that the hydroboost is allowing you to create excessive line pressure and distort the drums and shoes. The disc fronts probably give much less. Be careful you dont damage something. :)

Are you positive your brakes lines - front and rear - are hooked up to the Master cylinder in the proper position?

Many "ASSUME" that the front line goes to the front reservoir... and in some M/C that isn't the case.

Also - what else is in line with this system? Is the combo valve plumbed correctly?

The brake light is indicating a LOW PRESSURE situation.... could be air trapped where the switch goes in...

I can't imagine a hydroboost of any kind was designed for drum brakes.... I'm just putting my own "thinking" out here - but I would think they came after most OEMS had already upgraded their stuff to disc/disc. Maybe not.

I don't know why I didn't see your reply before now... :EmoteClueless:

Until you asked, I would say that I was 100% sure we had the lines in the correct place. Now I am only 99% sure. Same goes with the combination valve. I'll look into that.

The Astrovans used hydroboost with rear drums, and that's what the unit is off of. Still, someone else mentioned it may be the drums expanding. I'll have to look into that as well. I'm also wondering if larger bore wheel cylinders might help (if they are available).

Hmm...

I'm thinking I need to make a block off for the rear line, and see if the light still activates with it installed. That would eliminate flexing in the rear drums/shoes.