1.with the power bleeders:do they suck out the fluid(ala topsider oilvacs) or push the fluid out of the bleed screws at each wheel
Normally you start by sucking out as much old stuff as possible, with whatever is clean and handy. A small turkey baster works good. Then top the resevoir with fresh fluid. The actual bleeding part pushes the fluid through the lines and out the bleeder whether doing a pressure or gravity bleed.
2.regardless of wether they suck or push,doesn't the master cylinder become dry and if it does is that a bad thing
Yes and yes again. When bleeding the brakes you have to make sure the resevoir does not run dry. If it does you are then pumping air in the system - exactly what you don't want to do. If pressure bleeding make sure the tank has plenty of fresh fluid before starting the next wheel. If manually pumping the brake then top off the resevior every second or third crack of the bleeder screw.
3.if you go the gravity method do you have the key in the ignition to activate the pressure.hold on i guess the answer is no-that's why it's the gravity method.right?
Correct. However some ABS systems won't gravity bleed worth a darn.
4.my master cylinder appears to have two compartments- a forward and an aft one(why is that?).the front seems to be full yet the aft looks empty or real low.
Actually I believe the resevoir has three chambers. Yes the rear ones will go dry on you when you aren't looking closely. The fluid only transfers from the front chamber when the fluid is at MAX level, just pour the fluid in very slowly while keeping an eye on the rear chambers, you'll see when they are full.
5.why doesn't the front compartment leak into the rear to equalize the two?
See above answer.
Hope this helps.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.