Mityvac question....

[DervMan]

Hi all,

I tried using my new Mityvac pump today, it happy sucked all the old fluid out the master cylinder reservoir. I then connected it to the near side front caliper, opened the bleed valve and started pumping! initially some old fluid came out but after about 10 seconds no more fluid was coming out (despite continued pumping) and the fluid level wasn't dropping in the master cylinder reservoir (lid off).

In the end I bleed the brakes the old fashioned way, which worked fine.

Am I missing something obvious when using the Mityvac? The pump is working ok as I was able to remove the excess fluid from both reservoirs.

Thanks

 

[Pukmeister]

It was probably just sucking air in through the threads of the bleed nipple.

 

[DervMan]

It was probably just sucking air in through the threads of the bleed nipple.

That would explain it!

I'll try again tomorrow.

Thanks

 

[Smug]

Could be a collapsing flexible hose.

 

[DrFarkoff]

By any chance has it got ABS and Is It Pre 2006???

 

[Alan R]

I have changed the brake fluid on a few bikes using a vacuum pump and have found that there is a tendency to draw in air via the bleed screw threads. My thoughts are that the brake system doesn't normally allow fluid to be drawn from the reservoir unless the brake lever is operated and the vacuum will instead 'pull' air via the bleed screw threads.
The way I did it is to build up a head of vacuum against a closed bleed screw then, crack it slightly while at the same time, press the brake lever. This allows the fluid to be pressured out of the system and collected by the vacuum pump without any ingress of air or the need to open/ close the bleed screw at every stroke of the brake lever. When the vacuum head drops as judged by the gauge, close the bleed screw and build up the vacuum head again.
It works and just needs practice.
Alan R

 

[DervMan]

Could be a collapsing flexible hose.

Hoses are not collapsing. Thanks for the suggestion though.

 

[DervMan]

By any chance has it got ABS and Is It Pre 2006???

It's a non-servo 2007 with ABS, so uses the 'normal' bleeding procedure. I do have a GS911 to bleed the ABS pump too.

 

[DervMan]

I have changed the brake fluid on a few bikes using a vacuum pump and have found that there is a tendency to draw in air via the bleed screw threads. My thoughts are that the brake system doesn't normally allow fluid to be drawn from the reservoir unless the brake lever is operated and the vacuum will instead 'pull' air via the bleed screw threads.
The way I did it is to build up a head of vacuum against a closed bleed screw then, crack it slightly while at the same time, press the brake lever. This allows the fluid to be pressured out of the system and collected by the vacuum pump without any ingress of air or the need to open/ close the bleed screw at every stroke of the brake lever. When the vacuum head drops as judged by the gauge, close the bleed screw and build up the vacuum head again.
It works and just needs practice.
Alan R

Thanks Alan, air being drawn in via the bleed screw valve would explain the issue I'm seeing perfectly. I'll have another go tomorrow after wrapping some teflon tape around the thread and maybe not opening the bleed screw so far.

I like your suggestion of combining lever action while using the pump.

 

[Cook1e]

You've got it. I've been using Mityvac to bleed brakes for years. PTFE tape on bleed nipple threads is the way to go. Bonus is the nipple won't ever seize in the caliper. Take care not to block the holes in the bottom of the nipple.

 

[DervMan]

You've got it. I've been using Mityvac to bleed brakes for years. PTFE tape on bleed nipple threads is the way to go. Bonus is the nipple won't ever seize in the caliper. Take care not to block the holes in the bottom of the nipple.

Thanks Cook1e. ����