Air in Clutch Hydraulics

georg · August 15, 2015

After accidentally introducing air into the hydraulic system and bleeding it out by pumping the pedal (pump and hold the pedal with a piece of lumber, go under truck and open and close bleeder, release pedal), I will have decent pedal on my drive home. The next day I won't have full pedal and have to double pump the pedal to engage reverse or first. Bleeding again restores the pedal but the day after I end up with a soft pedal and have to bleed again. I have to bleed for about five days in a row before I end up with a permanent good pedal.

Exact same thing happened three years ago. I use my truck as a guinea pig to try out reverse bleeding, vacuum bleeding etc which is how I end up introducing air into the system, in case you are wondering why I end up with air in the system.

My question is, why do I temporarily have a good pedal after bleeding? After an hours drive and

sitting overnight, the pedal goes mushy again. Shouldn't the air left in the system immediately after bleeding not affect it right away, causing a mushy pedal?

georg · August 17, 2015

Brake m/c have the space behind the rear piston connected to the reservoir, so that it stays wet through the breather port. From what I see in the clutch m/c diagram in the FSM, this (non-working) space behind the piston is not connected to the reservoir, there is no breather port, so the piston is wet on one side only. Maybe that is why air is infiltrating?

georg · August 16, 2015

You use both Mityvac (vacuum bleeding) and pedal pumping? I thought that vacuum bleeding did not involve pedal pumping - just apply vacuum to the bleeder screw and wait for the bubbles to come out.

Doug_Scott · August 15, 2015

I use a Mitey-vac and a glass or plastic jar. Apply the vacuum to the glass jar, and run a line from the lid on the jar to the bleeder screw. You have to watch resevoir.

What you are experiencing sounds like what happens on some motorcycles when you try to bleed them. On a bike I use the same Mitey-vac and jar, but after bleeding I tie the brake lever back to the handlebar and leave it overnight. This somehow allows any trapped air to get back into the resevoir due to the port being open. I have had many say it can't work, but, I have more times it did work than times it didn't.