I have a 1969 cutlass with a new brake issue that I have been having a problem in resolving. I began noticing after driving my vehicle about 20 minutes, The front brakes were engaging. I have replaced the front brake hoses, the calipers, flushed the lines and replaced the brake fluid, (old stuff was brown)I took the master cylinder off and checked the pin, which is not adjustable. The master cylinder and power booster were replaced about 5 years ago. The issue appears to be heat related. I have parked it in the driveway running about 20 minutes and the front brakes are locked up. Let it cool and they are free again. I even tried backing off the bolts on the master cylinder and pull it away from the power booster. Same results. It appears that I have a distribution block for the disc/drum brakes. Do they go bad ? I have been driving the car for the past 5 years with no issue. HELP

 

How close are the lines and distribution block to the headers (if applicable) or exhaust?

 

They are in the stock position down on the frame and about 3 inches from the exhaust. NO headers stock manifold and dual pipes going back

 

What happens if you open the line at the MC when the brakes are locked up?

 

Just to be clear:
The problem is new without any recent brake work, correct?
And,
The brakes will seize up with the car just idling for twenty minutes, correct?

If a front bleeder screw is opened is there pressure and does that wheel release after the bleeder on the caliper is opened?

 

No recent brake work. When I crack open the bleeder at the master cylinder, it releases the brakes.

 

Then something may be wrong with the mc preventing the fluid from returning to the reservoir.

 

I have ordered a new one. It should be here next week.

 

I replace the master cylinder with a new one. Still having the same problem

 

Something may be preventing the brake pedal or the booster from fully returning to the retracted position. When the brakes are locked have someone pull the brake pedal to the up position while the cover is off the MC, watch to see if there is a surge of fluid from the holes in the bottom of the MC reservoir. If the piston in the MC isn't able to clear the holes in the bottom the brakes are held on. You should be able to see the edge of the piston seal move back and forth under the hole in the MC as the brake pedal is actuated. Hope this helps.

 

He's removed the MC from the booster with no change. Is the brake light on in the dash? If not do you know if the lamp is working?

 

It is not on and I am not sure if it works

 

You should see it illuminate if you engage the parking brake. Check the wire at the prop valve for the connection there.

 

I will check it. This has a distribution block and not a proportioning valve

 

I believe I have found the issue. I had a recent oil change that resulted in an overfill of motor oil. This resulted in excessive heat from the engine. I have changed the oil to the proper level and now the problem has disappeared. The oil level was about a half of a quart over full. After researching excessive heat if oil level was overfull. I found that it does produce excessive heat. The heat apparently effected the pressure in the brake system.

Thank all of you for your input

 

A 1/2 qt overfill will not cause that issue nor will it cause excessive overheating of the engine.

 

The consistent issue has resolved after changing the oil to the proper level. I would have never thought that it would have been the problem. I am estimating that it was a half of quart it may have been more.

 

Engine oil and brake fluid are two completely different systems. I don't see how lowering the engine oil level affects the brake system.

 

The whole problem was heat generated. under the hood, If I would start the vehicle with the hood down. in about 15 minutes the front brakes would lock up. I would open the hood and the brakes would free up after a few minutes. Even not driving the car and letting it sit there. I have changed everything to include front hoses, calipers, master cylinder, thermostat and replaced the brake fluid. The only thing that was done before this happened was top hose and thermostat replacement and an oil change that was over filled. The thermostat change did nothing. The oil change to the correct level alleviated the brake issue. Research on the internet indicates overfill on the motor oil increases heat. I changed it and it worked. After changing all the brake components, i figured it had to be something else. This is when I realized the oil change was done. I checked and it overfilled. It is the craziest thing I have ever experienced in a 60 year lifetime of working on cars.