So, I was looking through the auto store while picking up a mightyvac break bleeder vacuum kit. I noticed there was a syringe meant for mixing two cycle gas with a clear hose, and I thought "why am I paying $50 for this kit when I can just pay $5 for that syringe?"
Is there some reason just using a syringe for a vacuum bleed is a bad idea? I'm thinking I could put the hose between the syringe and the bleed screw just like I would with the mightyvac kit.
The only problem that I can think of is that the syringe has a fixed volume unless you use something like a check valve.
The problem is that it is easier to push the fluid out by pressing the brake pedal compared to trying to pull it out with a syringe. Your success may vary and it may be very difficult to operate it. In addition the small volume of the syringe is probably a limiting factor.
The cheapest bleeders have a 1 way valve on a pipe. They don't cost anywhere near what you mention. But it is possible to make your own bleeding tool easily, see this video
A suction device can only pull one atmosphere, which is about 14.7 PSI. No more than that.
A pressure device can push with more than 14.7 PSI, and therefore can overcome system resistance more easily. The effect is that the volume bled will be greater per unit time, and the job can be done faster.
Additionally, if the syringe drawing the vacuum is limited in volume, compared to the volume of the hydraulic system, some of the vacuum volume which will be used to bleed the system may be taken up in the limited, but not zero, elastic properties of hoses and other components.
Furthermore, a higher purge rate, obtainable with a pressure feed bleeder, will tend to evacuate air and other contaminants, due to turbulent flow, and other effects of a higher flow (flux) rate.
A very low flow rate for bleeding would be to simply open the bleeder on the brake, at the low point in the system. This tends to not work well, because some air and contaminants may not get flushed with a low flow.
In summary, using a pressure bleeder helps better purge the contaminants and has the added benefit of making the job go faster.
