Load compensator

[cwspellowe]

The front brakes were fine. under hard breaking with out a compensator the rear would lock up.

..but evidently not good enough. If you want a decent brake bias you want them locking up slightly before the rears, or for a bit of tail happy action the rear locking up first. If you've too much rear bias, you can either adjust the bias via brake pressure or by increasing the braking force at the front with calipers/pads etc.

I have no brake assists or bias valve and the fronts definitely lock before the rears.

And OP, depends how the compensator has seized, either on, off, or somewhere in the middle. A seized one means the bias can't be adjusted but isn't technically the same as not having one, with none at all the rear bias would be at its maximum iirc