Saturday, 20 September 2008

Bleedin clutch

With the gearbox under the DHC more or less sorted the gear change still wasn’t as it should be. So I decided to renew the clutch fluid, hoping that this would also expel some trapped air from the system.
So with some help from Rob we renewed the fluid and completely ruined the working of the clutch. One way or another air had got onto the system and refused to get out. As we were rather short of time I decided to leave the car there and then and return the next day.

Next day saw another failed attempt to get the air out of the system in the conventional way. In the end we concluded that this was probably caused by the fact that the clutch hose on a TR7 is rather long, as a result of which not all the air in the system gets expelled before the master cylinders reservoir needs a refill. During which time the air in the hose has more than enough time to collect at the conveniently placed high point just outside the master cylinder ...

(OK, this is no picture of the DHC, but the routing of the clutch hose is the same)

As the fluid coming out of the system was also rather dark looking I decided to put a new slave cylinder in (the master was rebuilt the last year, whereas the slave was a good used one).
Again bleeding the system proved to be quite difficult indeed, there remained some trapped air in the (top of) the hose. In the end we decided on a rather different approach: we waited a short while till the air in the system had collected at the top and then started pumping the clutch pedal several times. This revealed quite a few air bubbles in the reservoir itself and the fluid level started to drop slightly!
Due to the fact that the air collects right at the top pressing the pedal will compress the air, but it will also squeeze some fluid passed the trapped air. When the pedal is released the clutch itself will press against the slave cylinder which in its turn will move the whole (by now non pressurised) fluid column back up into the master cylinder. Every time taking small quantities of air with it into the fluid reservoir.

Five minutes later there were no more air bubbles and we again had a fully functional clutch.

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