97 Bravada brake issue

Recently our 97 Bravada has started popping the master cylinder cover loose when you apply the brakes. It hasn't leaked fluid, but it happens every time I put the cover back on.
Any ideas?
Thanks

Sounds like a clog in line..Or maybe cap clamps getting worn out..

Is the fluid level above the max fill line? The rubber gasket/bellows should compensate for air surges so long as the fluid level is not too high.