Any Tips for a Frozen Master Cylinder Cap?

[Grey Gunther]

I have not been able to remove the stock master cylinder filler cap on my M715 by using a crescent wrench and turning counter-clockwise with all the force I can muster with both hands. The vent cap (which is not vented in my case) on top of the filler cap unscrews just fine. I have not yet used Kroil or other petroleum-based penetrating agents, for fear of contaminating the brake fluid. Is there something I’m missing here, or should I accept the risk of breaking the cap and having to replace the MC? The brakes appear to work fine otherwise, but I have not put them to the test yet. Thanks in advance for any hints.

[MJCougler]

Well, you need to get it open. Obviously, you need to be able to check the level and periodically, you should suck out the old brake fluid and add new; flushing the lines is even better. That said, use the penetrating fluids and use the correct sized socket and a long handle breaker bar, or use a correctly sized wrench with a pipe on the handle. Besides, if you damage the cover, you now have an excuse to install a safer dual reservoir corvette master cylinder.

my $.02

[Barrman]

A 3/4" open end wrench should fit the cover. Yes, an 8 point socket will work as well. You can add fluid through the vent hole if you have to.

[moose3d]

And if you round off the cap get the ole pipe wrench to take it off.

[randyscycle]

If the filler cap is that hard to get off, imagine what everything downstream looks like.

Were it me (and this is just a friendly suggestion) I'd probably find this a great time to do a dual master cylinder swap, along with pulling all four wheels to see what the wheel cylinders and shoes look like, along with some new rubber lines and a good look at the steel lines too.

[maskale]

Yeah with a truck that old. Complete new brake and fuel systems are a must if you plan on driving with any reliability.

[Don Cavey]

Yeah with a truck that old. Complete new brake and fuel systems are a must if you plan on driving with any reliability.

And that is exactly what I had to do.

All new brakes from the Master Cylinder to the shoes, all four wheels. I wish I had taken pictures of the insides of the master cylinder and the brake cylinders

The left front brakes after new shoes, wheel cylinders and flexible brake line from AB Linn.



All new fuel system from the new tank to the rebuilt carburetor. This is what the carburetor looked like.





And the Sender:





I just knew in my heart that if I didn't address these issues, they would be dogging me for years to come.

.

[Mikel]

The best way is to unbolt the master cylinder from the firewall and throw it away. Then replace it with a dual piston unit.

[Grey Gunther]

Kroil didn't help--the MC cap is still frozen, so I'll check out the dual cylinder option and report back. Thanks for all of the replies to my post.

[Mikel]

This might be a good time to toss a booster in there as well. They can be had very cheaply at any junkyard and it will do wonders for your driveability...