Some Thoughts on Pressure Testing

[JZH]

If you've made-up a few brake lines from individual parts like I have, you might occasionally wonder if your handiwork is actually as safe as the professionally manufactured ones.  I've never made front lines (which arguably are the most important), but I have made up some rears (e.g., through the swing arm), including just recently on my project FP.  So I've been thinking about how I might pressure test braided stainless steel PFTE brake lines without spending gazillions...

 

Fittings are one issue, but fairly easily solved.  I can easily make up a set of fittings to work with banjo bolts and washers, something like this:

, which I mocked up to test an idea for putting a bleeder at the high point in my FP's brakeline-through-the-swing-arm set-up.  (It's an M10 x 1.25 threaded stainless steel spacer with banjos on either side.)  One such fitting could have a banjo one side and a pressure gauge on the other, and the other fitting could have a banjo and a fitting to allow the "system" to be pressurised.  If we were dicussing oil or water lines, that fitting would be a Schrader valve.  Hose fitting manufacturers like Earl's sell pressure testing kits comprising an AN- plug for one end of the hose assembly and another AN-plug with a Schrader valve for the other.  But brake lines need to be tested to 3,000psi or more--way, WAY too dangerous to do with air pressure...

 

So, after a little research I discovered that it is possible to adapt a common grease gun to create an "oil gun" with up to 8,000psi pressure.  In fact, there are commercially available "oil guns" which supposedly work better than converted grease guns.  (That last link might be this one.)  McMaster-Carr sells one, for example.  Most grease guns (and oil guns) use 1/8" NPT or 1/8" BSPT hose connections, too, which are easily plumbed into manifolds and fittings.  Oil guns are made for injecting lubricating oil into old Bridgeport mills, mainly.  But, if they work with oil, they'll work with brake fluid, no?  Using clean brake fluid to test a brake line would present minimal post-testing clean up problems (unlike oil or water, for example).  

 

What I don't know is if the pressure can be held steady using one of these oil guns, or if a check-valve and a purge valve (sounds messy!) might be required.  I haven't found anything on the web describing exactly what I want to do (which could be a clue!), but ideally I would like to make a testing apparatus for brake lines as well as regular AN/JIC braided stainless steel hoses and fittings.  Realistically, it would be more convenient to use air pressure for the low-pressure testing, so maybe I'm really talking about two different set-ups.  

 

Any thoughts?

 

Cheers, 

[Terry]

Why not simply assemble and bleed your full brake system, then apply some extra clamping force (e.g.a strategically located G-clamp) to the master piston to simulate a herculean squeeze? I presume you are only concerned about leaks, these would be obvious after a few hours of high pressure. Apparently talcum powder is good dusted around fittings to highlight leaks, 

 

You are correct that grease guns and the like can generate some serious pressure, but without a decent check valve I don't believe they will prevent backflow. And you may learn more about the "sealing" properties of the check valve than your brake system...

[JZH]

I hear what you're saying, but it doesn't really satisfy the testing criteria.  I don't know what pressure a Honda braking system is capable of creating, but it's nowhere near the 3,000-4,000psi the manufacturers use to check their brake line assemblies, so unless I discover a very good reason to use a lower testing pressure I'm going to use that as the standard.  Otherwise, I probably wouldn't bother testing at all.  As you suggest, leaks would be fairly easy to detect, but weaknesses in manufacturing may require the high testing pressures to become apparent.

 

I have an old mini grease gun I need to take a good look at, but it may be that I can simply lock the lever in position, or (what seems likely) perhaps the grease gun/oil gun design already incorporates a check valve?  Some do have means by which air can be purged (but I wouldn't want to use that to purge brake fluid into the air).  Hmmm...

 

Ciao,