I had all sorts of trouble getting my hydraulics up after my recent tear down. I had to do the front calipers and the clutch and neither were cooperative. Gravity was not my friend and based on my experiences and observations I challenge you to do an experiment using DOT 4 Synthetic BF. If you take a clear tube you can see why gravity will not work with this product. First, the fluid when confined to smaller spaces is like jelly. It's surface tension is so strong that it will not run down hill. It just sits there in the tube. You can partially fill a clear tube with DOT 4 and hold it up vertically with both ends open and the **** just sits there. It is this same surface tension what will not allow it to be pumped into new braided lines by the master cylinders. Another experiment: release the hydraulic line for the brakes or the clutch and put your finger over the hole where the banjo bolt goes into the MC. Squeeze on the lever and you'll shoot fluid all over. Remove your finger and squeeze the lever you get nothing. It comes out a little but as soon as you release the lever it sucks right back into the MC. It will move small air bubbles around easily if the space is mostly fluid and a little air. If the lines are new or have been drained the fluid will not displace the air. I gave up on mine and got a little trigger style vacuum pump and literally pulled the fluid into the lines and calipers. For the clutch I pressurized the system from the bottom up. The vac kit has the containers and fittings to make this possible although it leaked badly forcing fluid up into the MC - but it did work and worked very well. I've worked around brake fluid most of my life but I have never seen anything behave like this DOT 4 synthetic.

If you're having trouble getting the old-timey methods to work there is a cheat. If you have a shop that services several different kinds of vehicles you can probably scare up some DOT 3 brake fluid. You can use it to "prime" the lines but you can't leave it in there. This stuff is real runny and will work with the gravity method. Pumping up bare lines is still going to take a millenium so gravity might be the best bet. (the purests are wiggling in their chairs) Drain the MC as best you can with a syringe or by pumping it out with the lever. Fill with DOT 3 and begin your gravity feed - pour it into the MC just as the level drops away until you notice a steady stream out of the other end (caliper or slave). Allow the MC to drain nearly completely out. Close the bleed screw and fill the MC with DOT4 and start a normal bleeding process squeezing the lever and evacuating the air/fluid at the bleed screw. By the time the system is thoroughly bled you can be reasonably sure that the DOT 3 has been replaced by the DOT 4. I grant you this is not the preferred method but if you can't afford a $45 vac and need to get your lines back up it will work.