How accurate are brake bleeder gauges?

I tested my brake bleeder gauge against another vacuum guage, and I found the brake bleeder gauge read expotentually higher than the other gauge. I now wonder which pump, if any, is accurate.

Has anyone ever calibrated brake bleeder gauges? I got mine from LA Pumps, and I contacted them to find out if they calibrate their gauges, but I am still waiting for a reply.

We are using these gauges for something other than what they are intended for, and the accuracy is not as important for bleeding brakes as it is for PE, in my opinion. So, I think a note of caution is needed, even if you have a gauge.

The accuracy may not be right on, but at least they give you repeatability so you can reliably pump to a pressure you are comfortable with. Over the years I have used many of these gauges and I have never found any that I thought was way off from from the others I have used. I can pump up to a pressure manually and then switch over to my electric and get the same reading for instance.

You can do a crude test of your gauge easily.
Fill the tubing with water, draw in a bit of air and close off the end with your thumb. Measure the length of the air section (l1), pull a vacuum and re-measure the air space (l2).

l1 should be roughly equal to l2 times the pressure in atm (e.g. a 10”Hg gauge reading becomes 2/3 atm).

I just did this and landed only 6% off, which is a reasonable error considering the crudeness of my measuring (it’s hard to estimate the actual air volume with the connector in the tubing): 8cm -> 12.7cm at 0.667atm.
This should be accurate enough to tell you if your gauge is significantly off.

It doesn’t matter if they are accurate or not. You could replace the numbered dial with a piece of blank paper and just put pencil marks at the points that suit you. If someone says they pump at this vacuum or that vacuum, that doesn’t mean it’s the right level for you to imitate, so the numbers mean nothing. Feel means everything, and you can see when your penis stops climbing up the tube, or expanding in girth in the tube; work on noting those and ignoring the numbers.

More on gauge accuracy, precision, and how to calibrate using a DIY manometer here:

Anyone using water lift to measure/regulate vacuum?

We must have some science teachers on this forum. What a fun experiment.

I was going to reference Mr Wizard, but didn’t think anybody would know who he was.