What would cause soft brakes to get hard when engine revs?

1989 Jaguar XJS with no brakes - they go to the floor. Removed the ABS ECU fuse, replaced the accumulator, and now it seems the brakes come back up and get hard when the engine is revved to 3 or 4 grand or so. Keeping the revs up can almost keep the brakes hard.

Might have a vacuum leak in a line. Increasing revs would increase vacuum that is pulled overcoming the leak I believe.

Vacuum is not involved in Teves III ABS.

Maybe something to do with electrics/ battery. Sounds as though maybe it only pressurises the system with high alternator speed.Does your pump run as it should?

Big oops. Pulling the ABS ECU fuse and replacing the accumulator, I figured the ABS light should be on and I was wondering why I couldn’t hear the brake motor start up on power. Was a bad hose - brake system dry. Got pedal for some seconds, now. Two mechanics had looked at it, neither one apparently tried to put brake fluid in. What are the proper bleeding steps?

For what it’s worth, with little-no fluid in the system the brakes look like they can be ‘revved up’. In park revving to 3 to 4 grand made the brakes get hard for a while. Not ideal, but if you’re 40mph with a bust hose, might make a difference putting it in neutral and praying.

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I stand corrected, then sorry

All good, thanks.

Noticed a bit of a thing in the XJS Exp in a book. For brake bleeding it states head to 70.25.02 . This should be 70.25.03, at least according to my manual, which could be wrong, since it has it on page 70 - 48 ??

Correction: One of my manuals it is in 70.25.02, the other, 03
Either way, fingers crossed.