My 74 XJ6 lump has what seems like no brake boosting.

The booster has vacuum (tested it from the manifold, even dies when vacuum line is disconnected)

The check valve works and is in the right way (blew on it both ways to see its a 1-way valve)

I do not hear a hissing vaccuum leak at all, either in any lines or the booster.

The brakes work, but requires all my leg strength to do so. The pedal does not move or have any resistance difference with engine on/off.

Pedal moves a little bit (maybe 3/4 of an inch) then gets hard to where I'm the one doing the braking. Thinking this is more an "adjustment" thing than a sign of something working.

Car had been sitting for a while, so am thinking it could be old, bad fluid but would bad fluid alone cause these symptoms? Maybe a collapsed brake hose? But then, it does brake just with all my leg effort.

 

Maybe the brakes/ pistons have seized.
I would have initially said the brake booster seal but you have looked at that already.

 

Hmm, perhaps. Can't say I've looked at the calipers themselves, but I can rotate the tire freely so I'm guessing negative on that??

My previous lump had a leaking booster and that noise was fairly loud and obvious.. This car's engine is louder, but still can't hear anything leak-related.

 

A stuck caliper would still not explain a lack of boost. My money is on a faulty booster, if the vacuum lines check out as you say.

 

Another idea! Glazed rotors and pads. Even if boost is present, braking effort will not be felt. Low doefficient of friction means poor braking results.

In brake hoes, brake fade because of heat is partly that. Discs handle it better but are not immune from head indused fae as a result of glazed surfaces.

 

Its common that the master cylinder starts to leak out the rear seal and the fluid destroys the shaft seal in the booster causing a large leak. Sounds like you've covered the basics may be time for a new master cyl and booster.

 

Thanks for the ideas guys.. Am about to get a (free) replacement booster and see where that takes me. Don't see any fluid leaks around the M.C.

 

A fairly good test is when starting the engine. Press on the brake pedal with the engine off. Before start the brake pedal will be very hard, then start the engine, and as the engine starts and generates vacuum, the pedal should go softer, and the foot pressure will force it down more with the booster assistance coming in. If this doesn't happen the booster isn't working. However, if the vac lines are all up to scratch, (as you say), and you have vacuum at the port on the booster, it has to be a duff booster.

 

Yeah I did this, and there is no difference with engine off and engine on. I assumed it was a bad booster but I was just confused why I didn't hear a leak.. I thought a bad booster = a leaking vacuum sound.

 

UPDATE: I replaced the S2 booster with a working one from a S3 donor. Put the check valve in the right way, and the car had the best power brakes I've had!

Then, the same day while driving, I braked and felt instantly the assist die (back to how it was when booster wasn't working).

Thinking I blew another brake booster? Thinking I may as well replace the stock ('74) Master Cylinder while I'm at it. Might be causing the failure?

P.s. now theres no assist, and a whoosh in the footwell when pressed on the brakes. Pedal does not move with vacuum.

 

Try a new booster $$$. Any signs of a leaking MC such as, having to add fluid, softened paint or rust near the booster mount point? Lumps with a mild cam can make lots of vaccum on decel and may rupture an old diaphragm. It may be useful to take the time to see how each booster failed. Did they both fail on the rod seal? Check the push rod and make sure its smooth and not tearing up the seal. Did the diaphragms rupture on both? If so could be too much vacuum for 30 year old rubber. If you're in the USA it may be advantageous to you to get a booster from Autozone because of their liberal return policy.

 

BOOOSTER!!!

Your free booster may have been on a shelf for an extended period. thwn it's diaphragm was stiff with age and lack of use. Retrning it allowed iot to function for a time before splitting or cacking and thusly leadking vacuum with no assist.

I suspect a fresh booster will be the fix!

 

Don't see any signs of leaks from the M.C.. But it looks original and from '74 so I may as well buy a new MC too?

Yeah probably.. It was from a S3 donor car so no doubt it was old.