Help! brake failure

[shaggy n daphne]

Please help or no holiday for us, brakes won't bleed up so brought a new master cylinder part LM14037,not the same as the one i had so i put the new spring and plunger in my old one but it still blowing fluid back in the reserve so it's like pushing & sucking fluid, having trouble finding original part anyone know where i can find one my transit is a ci motorhome 1978 mk1 when you look on ebay they say bus,flatbed,van, What is a campervan?
Thanks Shaggy n Daphne

[DodgeRover]

Flat bed if its a coach built camper not a converted van with a pop top

[Mr RVM]

Ive got a few new master cylinders for older transits, could you post up a pic of your original one

[baconsdozen]

Seems an odd idea to swap spring and plunger.Post some pics of the cylinders.