No Escape from NADA Dual Line Brake Hell

Don’t mean to hijack, but I had no idea about the angled, third item in the bore, Mike :oops: Both my dual system V8s have been notchy and weird, meaning the boost doesn’t come on until some pressure has been applied to the pedal, giving a slight jerkiness all the time, between very light braking/holding and actual stopping power.

Using a NOS MC rebuild kit I found I needed to reuse the old ‘piston’ seal as the new one, while good, was much stiffer. This helped but not much.
 
I think I might have got my head around the system as a whole with the exception of the plate / lever that is strangely at the end of the MC that supposedly actuates the small piston. I was careful to drop it in and ensure it was seated in the slot upon assembly.

However I went back and disassembled the MC to investigate what this plate is actually doing. Here's the thing, the only thing that acts on the plate to move it and affect the reaction valve is the spring that is being pushed onto it by the piston / brake pedal. That spring is so weak that pressing it forward to simulate a small brake press imparts next to no force on the plate and therefore onto the small piston / rod that goes to the reaction valve. It will not overcome the friction of the seals of the tiny piston in the bore. So I am having a hard time believing this is what is supposed to happen. I had assumed that it was the pressure of the brake fluid that moved the tiny piston / rod to activate the reaction value.

Trawling the jaguar forums there is universal consensus that the plate operates the reaction valve when you press the pedal but I can't see how that is the case. I probably should be posting on the jaguar forums!
 
I'm with you on that....I think the reason may be that rod does have some piston value, enough to hold the stuff at the other end in position but not enough to actuate it alone. Then the plate restricts the flow out to the wheels when initially depressed causing enough pressure difference across the plate for it to push the rod in. Such a system would give much better feel at the pedal as it wouldn't activate on gentle pressures (ie. no boost required) but would activate quickly when the pedal was depressed hard and fast giving full boost.
 
Firstly, on the sticking brakes. After I removed the MC again and inspected the mystery plate / lever all was okay as I mentioned. I wrapped some emery cloth around a rod and laboriously cleaned out the tiny piston bore. After a while doing that, I slathered the piston in brake lube and it was a lot better with the seals not sticking it as firmly. I was 20% optimistic I could cancel my new MC order and it turned out I was lucky as the problem is completely fixed! Then I went for the longest test drive for at least 7 or 8 years at a frightening 2 miles.

I have a different theory on the mystery plate / lever that may be complete nonsense. I reckon it will only do anything if it is pressed on by the metal round tip at the end of the main MC piston. If you lose the fluid pressure in the MC and the pedal goes to the floor, which is not such a great scenario, the tip of the piston will hit the mystery plate / lever and activate the reaction value which will cause the servo to activate, therefore applying the front brake circuit. This is completely at odds with all forum chatter about the criticality of the plate / lever for everyday function.
 
You’re right. The plate is only there to allow the master cylinder to mechanically operate the reaction valve, and therefore the secondary brake circuit, in the event of a loss of fluid in the primary circuit. In normal operation, it’s the hydraulic pressure behind the tiny piston that causes it to move.
 
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