G valve issues?

G reg 90 with rear disk brakes.

So after my two year rebuild, i've been having a lot of issues with my brakes. Pedal becomes increasingly hard during a drive and eventually the brakes lock on. Its very sporadic. I can drive 50 miles using the brakes with no problem, then all of a sudden, on a long straight, i start loosing power and its clear the brakes are on. Come to a stop and i can hardly move the car while using just the clutch. Sometimes i jump into the car from cold and the pedal has a lot of travel. Othertimes i jump in and its already rock hard without pumping or anythign.

I've have a new servo NRC4772 and master cylinder NRC9529. New brake pipes to the front, but calipers and disks havent changed.

I assumed it was the push rod in the servo, so tweaked this over and over. Its now wound all the way in (allowing max travel of the pedal). But still, the system seems to 'pressurise' and the pedal becomes hard.

We considered the servo sticking (britpart!), so i replaced the new servo with another new servo of better quality. Still the same issue.

I've slackened the handbrake right off...just in case.

So only two things left- the master cylinder (new part) or the G Valve (i havent touched). As its sporadic and seems to build pressure, i'm guessing the G Valve. Would that be right?

Issue is, the G Valve (seems to be part number NRC8215) is not available anywhere! Why not? I cant even see any replacement part numbers.

@jamesmartin you must be the man for this one?!

Ive read that the G valve is when you have drums on the back and brake bias when have discs on the back. The brake bias is part NTC8836.
A G valve is a very simple device and apparently can be serviced as the ball can get stuck.
Though, I cannot see how this valve is causing your issue.

I would look into the vacuum pump and pipe, lack of vacuum can cause hard brake pedal problems.

The G valve wound be causing the issue of pressurising, it doesn’t work like that. It’s just simply a ball that moves under extreme hard braking to stop the rears locking up. You can take it apart and clean it if you like.

Have you a pad rubbing or anything like that? Sounds like heat is building up somewhere and pushing the brakes on it you have a partially seized calliper?

vacuum pump is ok - i actually swapped the whole engine out so it has a different pump and still same issue. Though potentially both pumps not working...
Servo was what i thought the issue was, so put in a new one. So doubt thats now the issue.
Master is new, but could be faulty.
I was considering being able to pressurise the back, btu the pressure isnt releasing, hence i thought it could be this valve.

Pads and discs all look good. If a piston was stuck i would surly have uneven wear and lack of brake rather than too much. It should also wear down the pad quickly until released if it was sticking in the 'on' position?

You need to Jack up when it happens and see what is causing the problem. This will tell you if it is whole system or just one brake.

The G valve is a ball that runs up a slope when you brake hard, when just driving around normally the ball is sitting at the bottom of the slope and doesn’t do anything. It doesn’t move with the normal fluid movement.

It could be gummed up and rusted in there but if it is rusted/gummed up enough to cause a problem then you have bigger issues as all that crap will be in the brake lines.

Are your wheel bearings okay, and diffs etc?

I am just thinking that it sounds as if something is getting hot as it’s running and then binding - could be brakes of drivetrain tbh