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this was a vh44 remote booster

That's a good point guys as we did have the master off while working on the engine and got it tested... (the master not the booster) That was the first practical run after putting it back in so I might have to get in there and investigate it!

Thanks again, fingers crossed it's that easy!

If the reaction disc has fallen out you would certainly know it. You would notice severe braking at the smallest pedal push. Like every time you brake your head would go through the windscreen but I sure do agree about the pushrod length. Easy way to find it is to push the brakes and if the front don't release crack a bleed nipple open and if it releases that's the cause. Master cylinder not returning. Either a cylinder problem or the pushrod too long. If the fronts don't make any difference do the same with the back brakes. You should be able to nail it.
The master cylinder must be able to return far enough to allow the primary cups inside to open a compensating port. This port/hole allows fluid to flow back into the reservoir as it heats up and expands plus visa versa. it is only very small but vital in releasing pressure from the hydraulic system.

That sounds spot on for the rod length. We mixed and matched a bit to get everything together so there was always the fear that something wasn't quite right. To get the car on the trailer at the track I had to bleed off some pressure from the fronts.

I know what I'll be doing next.

Cheers!

There's also some pedal height adjustment that may be working against you.