So the same thing happened with the original master and a new one? Too hot to touch is pretty unusual in my experience. I'm doing a thermostat breakdown of what GM designed these engines for and in general, engine temps in the 210°-230°F range is not out of specs. Even though you have a 427SB, the under hood temps should be in the "normal" range of what GM designed the compartment for.

Can you post a few pics of the engine compartment from each side? I have no idea what I'd look for, but am curious if something may pop out to someone.

 

Do you have the specs on the line lock? Perhaps post a part number and link, in addition to some photos? It may be rated to handle brake fluid, but perhaps the servo or relay can't handle C3 underhood temps.

 

The line lick is nothing fancy https://www.jegs.com/i/JEGS/555/6300...hoCjwkQAvD_BwE

As far as the engine bay it's all pretty stock looking minus the long tube ceramic headers and electric fans

 

H-m-m-m-m-m-m- I say.

Sorry, I don't like the looks of that Line-Lock. A lot different design that the traditional roll control, Hurst, etc.
Not an expert on Line-Locks, but I checked into them a couple years back. Seems like a couple hundred bucks for anything with a good reputation and even a hundred bucks for a used one.

I know you have it disconnect electrically. But as I said earlier, I think that Line-Lock has turned into a "One-Way-Valve" for unknown reasons.

 

So apparently this vette had this issue back when I bought it for 3000 before the restoration, go fast goodies and more importantly installation of the line lock.

In any case I just pulled the trigger on this https://www.ebay.com/itm/14392370847...media=IG_STORY

I have way too much money invested in this vette to have it undrivable

if still no luck I'll ditch the line lock

 

Or get a better one. Your present unit seems really cheap.

Is there a way you can by-pass your current Line-Lock for a road test?
Of course removal of any brake line will introduce an air bubble. So you know what that means.

 

I agree with investigating the line lock, but I have a Hurst unit with no problems.
Why don't you take the hood off and see if the problem occurs?

 

Well without a doubt that would fix it, underhood temps are definitely what is causing the issue or triggering them rather.

I wouldn't be having any issues at all if it was 60 degrees or colder outside.

the only brake components which aren't new since I bought the corvette which already had this issue is the MC and booster so i just bit the bullet and ordered those two.

 

xzero117
I think 66:72 has a point worth further exploration regarding the power brake plunger/master cylinder pushrod.
I also would be suspicious of the line lock, butttttt........

You might look at this from the opposite direction, it may not be the heat that is causing the brake problem, it could be the brakes dragging that is causing the overheating AND the subsequent lock up of the brakes. Several years ago I had a problem with brakes locking up on a C4 to the point i had to stop for awhile and then they would work for awhile and lock up again.

The culprit was an incorrectly adjusted brake master cylinder pushrod that would not let the piston return far enough to let the fluid back into the intake/return port. Each time the brake was pressed, more fluid would be pushed to the brake caliper and could not return because the return port was not fully open. By not letting the fluid return to the master cylinder the brakes never fully released from the rotor. This finally would cause the brake to lock up. Given the torque of your 427, it could be overpowering the brakes to the point of boiling the brake fluid and final brake lock up. Once sitting for awhile the cooling and fluid slowing flowing back into the return port to the reservoir allows the brakes to function again.
Of course if the return springs or seals could be bad which would result in the piston not returning but given you have used a couple of different master cylinders that might be less likely the problem.

See the attached diagram and description of how the master cylinder functions regarding fluid flow to and from the brakes.
https://learnmech.com/master-cylinde...-applications/
Good luck
Gary
PS It would be worthwhile buying a temp gun to measure rotors, calipers, lines and master cylinder temperatures.

 

Adjusting the pushrod on the master cylinder "pushrod" is critical as stated. Before you take it apart you can test if it is adjusted correctly by just taking the M/C cap off and making sure it squirts when the pedal is released.
#2 the "combination valve" is also a source of issues on brake jobs. Search for a video on how it functions, and how to set it properly. You need a little $8 tool to hold it still while you bleed the brakes. If not it shuts off all flow to one end of the car. Test the brakes on a lift and see if both front and rear are applying braking force.

 

It's probably not the line lock. I'm dealing with the same problem and I don't have a line lock.
I removed my thermostat and wrapped my mc with welders cloth, and the brakes stopped locking. Then I removed the welders cloth and they still didn't lock. I talked to my engine builder and he said the stat could be too restrictive and causing the engine to run hotter than it should. He recommended installing a 160 degree stat, so I did. He also recommended moving the temp. sender from the head, to the intake, next to the stat housing, and I did that too. I had to order a different stat housing to accommodate the sending unit height. I drove the car for about 30 minutes and had no brake locking. The engine builder told me to expect the engine to run 15 to 20 degrees above the minimum stat temp, the temp now averages 180 to 190, I too added a few go faster goodies.

 

@xzero117, did this ever get resolved?

 

It was the booster, it was the last brake component which was still original after the restoration, well besides the distribution block.

I'm guessing the scorching hot underhood Temps made the booster stick a bit and eventually lock the vette solid as if you were standing on the brakes.