wrong brake servo or too much vacuum ???

[crypto]

After a few miles driving and braking/stopping my brake servo brings my Elan Sprint to a full stop without pressing the brake pedal ! The car will not move anymore as the wheels are blocked. When I turn off the engine and remove the vacuum hose to the servo, the brakes are free.

The servo looks new and when I bought the car I always had the feeling there is no brake servo assistance but it never blocked the brakes as it does right now. Now, the restoration of all the rest is finished and I wonder if I messed up something ? The vacuum on the pods seems also ok and I don't want to remove the servo.

Any ideas ?
Thanks, Peter

[rgh0]

My guess would be the servo control valve is sticking

cheers
Rohan

[vxah]

Have you checked the master cylinder pushrod? My thinking is that if the pedal return is a bit tight the heated and expanded brake fluid will not be able to return to the reservoir? The build up of pressure could start to make the servo vacuum assistance work and shove the brakes on.. harder and harder as the fluid gets hotter and hotter. When you remove the vacuum supply the pressure in the system, now transferred to the master cylinder could be enough to push the piston back dumping the excess fluid into the reservoir?
That said of course the servo could be sticking and causing the same issue, had it on a Girling Mk2 B although not to that extent!

[Hawksfield]

Hi

Is it a Girling MK2B, as the control piston can stick if the secondary and primary control seals have swollen
these parts are available on fleabay also the vacuum control valves

See northeast classics

[richardcox_lotus]

Sticking Servo is not uncommon, especially if it's not been used/ sitting on a shelf and all the seals have got somewhat dry.

[alan.barker]

if the master cylinder was replaced and the old push rod was not used it could be the problem. The push rods on new master cylinders are too long. If this is the case you can put shims between master cylinder flange and pedal box to give some free play or shorten push rod or find old push rod
Alan

[crypto]

Gentlemen, thanks for all your hints and advice.

I'm just a little closer to the solution, but had no time to go for a longer ride to see if its really solved.
I checked the push rod on the master cylinder, but there is some play between brake pedal and rod but maybe not enough, I will re-check when I go for a longer drive.

What I found out is the vacuum non-return valve was fitted the wrong way around. My impression was right, I never had servo assistance. I also removed the cover of the air-valve diaphragm and everything looks new and ok. But why can it block my brakes ?

The one a I have is the not the Girling but the Lockeed type of servo.

Internally, it looks about like this

[bitsobrits]

My thought would be to ditch the servo. 1600lb cars don't need a servo.

[david.g.chapman]

According to the Lockheed installation info, the air valve should be facing down at a 45 degree angle. I can't find my sheet to attatch it, but there are other copies on this forum if you search.

It might not cure your problem but it could help.

My Lockheed servo has been working in my +2 for 9-10 years now. Not quite as good as the original Girling, but certainly OK.

Running without a servo is an option, but you might want to change the master cylinder diameter. Again check out the forum with a search.

Cheers,

Dave Chapman.