Brakes lock up once warm! No idea what the issue is.

[OldSkoolFool]

Car is a 1971 240Z with stock components.

Once I drive the car for about 10 minutes and it warms up, the brakes start to drag and continually get worse. If continuing to drive, it gets to the point where the brakes drag so much that the car is unable to drive. The front and the rear brakes both drag. Once this occurs, I stop the car and bleed both the front and rear bleeders on the master cylinder. Fluid comes out. The brakes then work fine and I can continue to drive without the brakes dragging. If I stop driving the car for multiple hours and let it cool down, I have to do bleed the master cylinder again once the car warms up.

I have spent hours and hours on this issue and am at my wits end.

1.) Assume, as if I have done nothing mechanically, what might be the issue?

2.) Now.... below is what I have tried:

-New master cylinder (installed by previous owner)

-Different master cylinder (from my parts bin)

-Combined parts from the "different" master cylinder and the "new" master cylinder as the front seal seemed tight on the new one (this is what is on there now)

-Bled the brakes

-Different vac booster (from my parts bin. Pedal used to have a very odd feel to it. It would be super sensitive and easy to move for about 1/2" then after that rock hard. My parts bin booster has solved that odd pedal feel.)

-Different front calipers (from my parts bin)

-Adjusting the rod/piston further in and out on the back of the booster (so the master cylinder guts travel further or not as far)

[jmortensen]

The port to the master cylinder reservoir isn't opening when you let off the brakes.

This is either caused by the pedal adjustment being too tight (should be 1/8" or so free play in the pedal before the master cylinder starts to move), or the rod in the booster is too long.

When the master cylinder port opens to the reservoir, it releases whatever pressure might be in the system. If it cannot open then as you drive the fluid gets hot and expands, this is like stepping on the brakes a little tiny bit. This causes the fluid to get hotter and it expands more which applies the brakes more and creates more heat etc and is a vicious cycle until you pull over with smoke coming out of the wheels. If you cracked a bleeder open at that point, it would squirt out like someone was standing on the pedal.

Get some free play in there and you'll be good to go.

BTW I had this same problem with a clutch. Went to the track and it would work fine, but at the end of the session it was slipping. Checked everything, no leaks. Bled the clutch, worked fine again. End of the session, it was slipping again. Same exact thing. I had taken out all the slack in the pedal adjustment (plus I guess an extra tiny little bit that closed the port to the reservoir).

[OldSkoolFool]

Jon, Thanks for the reply. So you would say the master cylinder is most likely just fine?

I have tried adjusting the pedal before with the new/1st master cylinder but do not think I have done so with the current set up so that will be my first step. How can you tell there is 1/8" free play in the master cylinder? As the pedal is adjusted to the floor, the rod in the booster that comes in contact with the master cylinder goes towards the master cylinder or away?

Thanks for the help!

[jmortensen]

It's been a while since I've been into the stock pedals, but there is a clevis that attaches to the pedal, and then there is a threaded adjustment between the clevis and the booster. That is one adjustment, and the pedal should move 1/8" before the brake booster part comes into contact. Then there is the pin inside the booster, which I think has an adjustment on it too.

I have dual masters now so I'm going back 10 years or so to think of these details, so forgive me if I'm a bit fuzzy.

[Oiluj]

Had this same exact problem on my MGA about 4 years before I sold it. I wish Jon hed been there to explain it to me then.

Took me several weekends messing with the all the brake components before I figured it out...

[beermanpete]

I recently went through this procedure on my car. There are 2 free plays to check. There is free play between the pedal and the master vac, and free play between the master vac and the master cylinder. You should be able to detect both. Each is adjusted separately.

Here is the factory service manual for the '72 brakes.

BR Brake System.pdf

The brake pedal free play is not specified in the factory manual per se. The manual says to set back off the pedal stopper and then set the push rod length to attain a pedal height of 8.11". Next, adjust the pedal stopper to attain a pedal height of 7.99". Also, make sure the brake light switch is not intering with the pedal stop.

The second free play adjustment requires removing the master cylinder to adjust.

There is another possible cause of your symptom. The bleed hole(s) in the master cylinder could be blocked. The remedy for this is a master cylinder rebuild.

[240260280z]

I have read that hoses can age or clog and act as 1-way valves causing similar symptoms.

[OldSkoolFool]

Thanks for the replies. I'll start first with the two areas of adjustment and see how that goes.

As the brake pedal is pushed, the clevis and rod that is directly attached to the pedal goes in towards the front of the car or back towards the rear?

[twsutt]

As the brake pedal is depressed, it pushes a rod into the brake booster (toward the front of the car). The booster uses vacuum to provide a lighter pedal effort. On the engine side of the booster, the other end of the rod that is connected to the brake pedal presses into the Master Cylinder. This end of the rod is adjustable. *Watch out for the dreaded reaction disk.*

[SteveJ]

While the 71 FSM isn't readily available, it would be in your best interest to download the 72 FSM and read the BR section.

Here's a picture from the FSM that will answer your last question.

[robftw]

I had this same problem about a month ago, i replaced everything in the brake system.. It turned out to just be the pedal adjustment. Under the dash you will see a stopper for the brake pedals return distance, back this all the way out so the pedal has a large amount of free play and take it for a test drive, see if that fixes it.

Afterwards adjust it as necessary so that it is safe for everyday usage, IF this is not the cause it is probably your flex hoses between the caliper and hard lines to the body they wear out and will eventually create a one way valve on the inside.