Intermittent brake pedal on 924Turbo - master cylinder?

[retroracing]

So bled the brake son my 79 924 turbo at the weekend after replacing a rear caliper. (rear discs)
Had a helper doing the pedal to do it the proper old fashioned way.

Brake pedal all good and firm with no creep. HOWEVER, the first time you hit the pedal it goes straight to the floor, before you get the pressure?? It then hits the floor every do often.

In the reservoir, looks like a jet of fluid hits the top when pedal depressed?

Could the master cylinder be sucking air? Id like to try and figure out the problem before ordering a new master as they are a bit pricey,

Thanks