Bench Bleeding a Master Cylinder ?

[pancho400cid]

So…. It’s been a while since I replaced a master cylinder. Last night I picked one up at an Oreilly’s near me for a 78 Pontiac Trans Am.

I’m used to getting plastic bleeder fittings with hose barbs and short lengths of clear hose for bench bleeding. The hoses got stuck back into the fluid in the reservoir. You just pushed the piston manually and the air got pumped out – very straight-forward. You could “see” the air get pumped out.

This unit came with plastic plugs – no way to hook hoses to them. The piston pressure apparently just dead-heads against the plugs and this apparently lets the air get worked out. I followed the directions and the piston did firm up when I bled it on the bench, but I dunno…. I’m kinda leery of it.

Can I trust the “plug” method?

 

[texasprd]

If you don't trust that method, get this - other versions are available with more fittings or specifically metric or SAE. I have one I bought years ago and bring it out when I need to bench-bleed a MC

 

[JasonMcElroy]

I like to use vibration when bench bleeding (or even bleeding in place if accessible.

Talk a small orbital sander and press the body up against the side of the mc.

If you're the curious type, do this with the top off and watch how many small bubbles appear and rise to the top.

Jason

 

[SgtHawkUSMC]

The last one I put on my 79 Z28 came the same way. Luckily I kept the hoses and fittings from the last time an MC came with them. I don't know if I'd trust it either. You could easily make a set that would work. Grab a couple fittings, roll a couple 8" or so lengths of brake line, flare them and away you go. Come to think of it. I'm going to do that for next time lol. I hate the little barbed fittings and plastic hose. They always leak a little.

20171022_121725 by Dave H, on Flickr

20170919_135248 by Dave H, on Flickr

 

[midwestcj]

yup. just cycle the plunger and it'll work fine. just did that a couple months ago on my Jeep MC.

 

[jsaw]

I just bolt it into the car and have someone slowly push the brake pedal while lines I have the lines cracked loose. Then tighten and have them release the pedal. Usualy only takes a few pumps before you get just fluid/ no air. I have used the plastic fittings and hose sometimes supplied with master cylinders, but more often than not, the hoses blow off the plastic fitting and end up making a mess and take longer than if I had not used them.

 

[pancho400cid]

If you don't trust that method, get "this"

Too obvious...

Actually looked at Oreilly's and Autozone across the street and neither had a kit like that.

The last one I put on my 79 Z28 came the same way.

I believe that is the exact same MC I'm dealing with.

I didn't try the sander for vibration, but did tap the MC body with a brass drift punch a number of times and definitely had air bubble up into the reservoir.

In the long run I just used the plugs and I'm hoping for the best. The whole system needs to be bled so we will see how it goes. Definitely got the piston firm and no air coming up into the res now.

Thanks for the input!

 

[gtr1999]

I work on old vettes and what I like to do is get the short steel lines NAPA has or had the last time I needed them. These are metric, I was surprised to find a MC with metric threads in a 69 vette but that is what was in there. I cut off the flared nut on one end and screwed in the lines on the MC. Then bent up the lines into the chambers for a solid tight connection, bench bled and installed. Works great.

 

[bwringer]

I think they're just saving the 12 cents or whatever. I'd still prefer to bench bleed.

I've done it before using tubing with an OD that happens to fit the outlets tightly. Just screw it in to the ports.

If you want to get fancy, you can form most plastic or vinyl tubing into a loop shape by sliding it over a piece of copper water line formed into the correct shape, then dipping it into boiling water. When it cools, slide the tubing off the copper and it will keep the shape.

I've also done a quick and dirty bench bleed in a pinch by just using my fingers as one-way valves. Cover the ports with your fingers, and the fluid and air will squirt out around your fingers as you push the piston with your other hand. Just keep your fingers in place and don't let air in on the return stroke. Messy, of course, so put down a pan or paper towels.

 

[metlmunchr]

I bought a m/c for my 92 F-350 at Oreilly a couple weeks ago and it had the same plugs. I asked the counter guy why plugs rather than the plastic fittings and short hoses.

He said they'd received info from the supplier that they went to plugs because too many people were shoving the piston to full stroke on a basically dry cylinder and ruining the seals rather than following the bleeding instructions that tell you to start the bleeding with short strokes like 1/4" and only go full stroke once you're getting fluid thru the hoses and back into the reservoir.

I bench bled mine with the plugs, and then switched to the adapters and hoses I'd saved from other cylinders to see if I got any additional air. Obviously, it pushed the air out of the hoses, but I didn't see any additional small bubbles once the hoses were cleared, so I assumed the bleeding with plugs does work.

I would say its much more critical that the m/c be held level when bleeding with plugs since holding it at an angle could trap air that may not be able to vent back thru the holes and into the reservoir. No such problem with trapped air when using the adapters and hoses since you're not counting on all the air venting back up thru the holes into the reservoir.

Re the 69 Vette m/c, the larger port is 9/16-18. A M14 fitting is about .011" smaller in diameter, and the pitch is within .006" of being the same as that of the 9/16. The smaller diameter and short engagement length allows the M14 to screw in far enough to tighten the flare to the seat.

Not saying some bastardized third world replacement doesn't have an M14 thread, but only that that's not what was original. I've owned a 69 since 1970 and am pretty familiar with every nut and bolt from one end to the other. The way to tell for sure is by trying to thread a 9/16 into the hole. While a M14 fitting will screw into a 9/16 threaded hole, a 9/16 will not screw into an M14 hole because its too big to start.

 

[pancho400cid]

Lots of good info above.

My car is finally done. Brake job from hell including fabricating two brake lines.

For the record, I agree the plugs work if you follow the directions...

Even so I may still buy one of those MC bleeder kits. The old ways die hard!