All of a sudden the brakes are "hard" to depress, they stop in a straight
line but take a lot of leg pressure. No fluid leaks anywhere in the whole
system. It is a 1974 SII XJ that is having the problems...

I confirmed the one way check valve is working correctly and installed with the
arrow pointing down the hose...the hose to the reservoir is fine and the
check valve on the booster is working...

Anything else to check before I change it out?

Also, will a 1984 SIII brake booster work (it has a blown V 12) as it will
never run again...

 

Roger,

You have about covered it. That valve under the front carby is the most common external cause.

Old brake fluid will give similar symptoms, as in a rock hard pedal, and the cars unwillingness to slow down is fluid related.

The S3 booster will fit just fine. The master cylinder will be different as in the container, and metric pipes.

 

Or, the booster has a massive vacumn leak and therefore affords no boost resulting in a rock hard pedal. But, using lots of leg power should result in braking!!

Disconnect the engine to boost bellows and attach a mitty vac. Pump up a vac of about 15hg's. Just a guess. See if it can be done and if it holds for any period at all.

Carl

 

Boosters usually fail gradually so Im supprised you got a rock hard pedal all of a sudden. Do you hear a whoosh sound under the dash when the engine is running? Also make sure you have good vacuum after the check valve. The 3/8 hose should offer up obvious vacuum and that amount of air bypassing the carb should make the engine stumble. I usually use a A/C vacuum pump with a Vac guage to vac the system and listen for leaks.

if your booster is bad...Check the back of the Master Cylinder for leaks. Typically the rear seal on the MC starts to weep and the break fluid destroys the one or more of the seals on the booster. Usually there is a tell take streak of rust and removed paint on the booster where the MC mounts. If your MC is leaking you'll destroy the relacement booster quickly.

Check that the Series 3 you have as a spare holds vacuum before you remove it. No sence is doing work to remove a bad booster.

 

How quickly would a booster fail if being destroyed by a leaky M.C? My donor replacement failed instantly while driving, a day or 2 after I replaced it and had it working.

 

There's no way to know for sure. How bad is the Leak? Was the replacement booster new? A new recently re-manufactured unit with new seals will last a few month or longer with a leaky MC. A older used unit, disturbed by removal and installation with marginal seals may last a few days with a leaky MC contributing to seal degradation and then pushing it over the edge of failure.