QuestionQUESTION: Hi Barrie: I have an '80 MGB made for the USA. It was time to bleed my hydraulic lines so I invested in a manual vacuum pump. This was new to me; previously I alway enlisted a "pedal pusher." The brakes went fine but the clutch bleed has resulted in a strange result. I started by manually pushing in the slave cylinder rod to clear out any air in the master cylinder. I then attached my vacuum pump and first drew a vacuum to verify that I had no leaks. Next I opened the bleed screw and "pulled out" hydraulic fluid and lots of air bubbles. I filled the reservoir three times. The fluid was clean but the air bubbles would never stop. I finally gave up and checked the clutch. It felt fine. Went for a drive and all the gears shift just like they alway did. There is no sign of air in the hydraulic line. My question is where are all the air bubbles coming from??
ANSWER: Hi Bob. A good question. Did you clear out all the old fluid first? Did you re-fill from a new, sealed container?
Old fluid gets saturated with water from the air around you. When you apply a vacuum, air trapped in the moisture will separate out and you will see this as bubbles. Clean fresh fluid from a sealed container will not contain these bubbles. Normally, the fluid operates under pressure so the bubbles are not visible. Just like opening a bottle of Champagne.
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QUESTION: That is an interesting point! I had not thought about the difference between increasing the pressure by pushing down on the clutch pedal and reducing the pressure by drawing a vacuum. First, as you suggest, it would increase the volume of a compressible fluid (air). Second, if there was a leak at the bleed screw there would be a pressure differential that would introduce air into the hydraulic fluid outside the hydraulic line. While my Castro hydraulic fluid is well over a year old it was sealed until a week ago. Even in my hot humid Florida garage that should not introduce a significant air content. Besides I did not see this air in the brake lines I did last week. I need to find a "pedal pusher" and repeat this process using the conventional approach to see if there is a different result. Thanks for the insight.
AnswerHi Bob. I just gave Steve some advice about bleeding his MG Midget clutch. The same advice applies to your MGB:
Unbolt the slave cylinder from the transmission, and just let it hang vertically on its flexible hose. Push the piston up into the cylinder with a Philips screwdriver (or your thumb) to expel any air upwards. This works much better than using the bleed screw or a vacuum pump. Make sure the master cylinder is topped up to the correct level afterwards.
By the way, a vacuum pump can cause the cup seals to collapse, allowing air past them. This is one reason why I don't like vacuum systems. The system is designed to be under slight pressure even when your foot is off the pedal, so the seals remain expanded, and they should not leak or suck air in normal use.
