Sinking brake pedal... aaarrrggghhhh

[andyburns]

Hi guys, I am having an issue with my 67 250s break setup and would appreciate any help on offer.

I am in the final stages of restoration http://www.flickr.com/photos/andyburnsnz/ and have replaced just about everything, including a the brake booster and master cylinder with brand new factory components.

I have an issue with sinking pedal and imediately put it down to a faulty 'brand new' master cylinder. I bit the bullet and brought a new one and had it tested professinaly prior to installation. The old one 'new one' that came out showed no signs of leaking out the back seal and the booster is dry. There are definitely no leaks anywhere in the system.

With a disconnected vacumn hose I bleed the brakes up with over two litres of fluid to ensure there was absolutley no air in the system. I got a rock solid pedal with no boost assist.

With the vacum connected back up and the car running I get the sinking effect almost all the way to the floor. I can here a very distinctive 'hiss' from the booster from the inside of the cabin.

When the car is turned off the sinking continues for four of five more depressions of the pedal until the vacumn is expelled from the booster. With each press the sinking deminishes until when there is no vacumn you are back to having a firm pedal again.

The only thing I can think off is that when the booster is operational with working vacum that the force applied to the master is much much more than you could apply unassisted and that perhaps I have a bulging or swelling brake hose which is faulty only under increased pressure due to the effect of the booster.

I am driving myself crazy with the problem as my car isn't warantable like this and is at the end of a two year resto. All I want to do is drive it!! Any help or advice appreciated.

I can supply the details of the break booster and master cyclinder part numbers if required. I have wondered reading other threads if I have the correct parts.