New Servo Locks-up brakes?

I purchased and installed a new Brake Servo (R.N.) for my 68 2a (dual circuit master cyl.) I did not replace the working master cyl .working fine, good pedal pressure, no loss of fluid, no bleed-down, wheel cylinders are not leaking. All worked well for a couple short runs, good power pedal assist and good braking, I have recently adjusted all the brakes to work well.

While on a longer road trip, pulled over the the brakes were locked up and could not move the vehicle under power. Checked emergency brake - nope, working fine. I disconnected the vacuum rubber hose from the carburetor base plate (also R.N. Weber conversion kit), to the Servo, rocked the vehicle and heard a clunk (Servo spring released). Ran the vehicle on the manual brakes fine just a hard pedal but working fine to stop. I connected the pipe again and same lock-up issue. Disconnected now and running again with no servo assisted brakes with good pedal movement.

Why did I remove the replace the original servo? It was a rusty mess, the plastic vacuum pipe fitting was broken but actuallly it worked better than the new one - inoperable now.

Thanks for the thoughts. Wondering about installing a vacuum regulator in the pipe to reduce the vacuum?

MT Bugeye

Attachment 14110

I would say the either the valve or main piston is sticking in the servo or the pushrod between the master cylinder is the wrong length. I had exactly this issue with a servo on my old Rover car , the main servo piston was sticking on, and it meant the fluid in the hydraulic circuit could not return back to the reservoir, plus as the fluid gets hot with brake application it gets worse & eventually locks the brakes on. I would say trying to reduce the vacuum will not help. The mechanical issue needs rectifying. Presumably there is nothing stopping the brake pedal fully returning. As a matter of interest , why did you replace the servo?

Thankyou roverp480, sorry for the late response. Advise sounds logical. I have several issues with Pro-Line parts from R.N.not working or fitting, etc. I have a couple other servo brake cars and I agree it is likely the Servo. Master dual circuit system works fine without servo assist, just hard pedal that depresses about 2" and holds good. The original servo (about 54 years old) was rusted with brake fluid seaping out the bottom rim, the plastic inlet vacuum hose fitting was broken with bad seal. This is a preservation restoration but I do just replace some parts when worn badly. My Sunbeam Tiger Girling Servo costs $700-900 to rebuild so, I just pitched the old Rover canister, probably should have saved it.

I suggest you check & see if your snail cams are over-adjusted. This could make the hydraulics over-pressurise the system & not allow your master cylinder to release pressure. Does that help ?