My brakes are grabbing

[Jackho]

75' Spider veloce RHD, dual booster brake system.
By the attitude of the car when it happens the brakes are grabbing on the rears .
Almost every time I brake after they have warmed up for a couple of minutes I find them binding slightly and if I press the pedal moderately again they let go.
Brake to the lights, car wont roll freely.
Press the pedal once, brakes let go.
Is this likely to be a valve on the booster as someone has suggested to me?
How hard is it to rebuild a booster unit?
Any help most appreciated

[ian]

I would reckon it is a problem with one of the boosters. Not too difficult to rebuild if you can get the seals. Most likely the top seal binding on the rod.