Bench bleeding MC with plugs

[Apogee]

In general, I bench bleed master cylinders using the plugged port method for two reasons.

1. I receive positive feedback when all of the air has been bled out of the master cylinder in the form of a hydraulically locked piston after a very short stroke. How far it will stroke will be determined by the distance from the cup seals to the transfer ports into the reservoir(s), but it's usually 1/8" +/-1/16" or so in my experience.

2. Once fully bleed, I can apply force for a period of time to verify that it doesn't sink over time, something that would indicate an internal seal bypassing fluid under pressure.

I've been using this method somewhat exclusively for ~20+ years due to a spate of bad experiences installing faulty remanufactured master cylinders onto various vehicles, my '77 EB being one of them. I remember bench bleeding one Napa master cylinder for almost an hour with the recirculation method only to find that one of the internal seals was faulty after putting it on the vehicle and then struggling to get a good pedal for the better part of another couple of hours. At that point, I decided there had to be a better way, and IMO, there is. The recirculation method moves fluid from the bore back into the reservoir, but it does so at atmospheric pressure, so any bubbles trapped in seal grooves or in tight places don't necessarily have any motivation to release from whatever surface they're clinging to and move with the fluid in my experience. FWIW, both methods are "approved" methods by most brake hydraulic manufacturers.

On a side note, I reserve reverse bleeding techniques for ABS/traction-control equipped cars where you would otherwise need a computer to overcome the hydraulic control unit...and for my brakes and dropper seat post on my mountain bike.

Tobin

 

[ba123]

Yeah, the plastic stuff is really junky. And unlike many of the things we deal with today, it has been for more than 40 years!
You need to either be very lucky, finicky or clever, or just make your own from hard fittings and tubing.

Paul

Like I mentioned, I tried one, they sucked and then tried another. But the second round was perfect which is why I try to keep mentioning g which ones did work so that if anyone does it, maybe they can start with those orange ones and have it work the first time.

 

[ksagis]

but it's usually 1/8" +/-1/16" or so in my experience.

Really appreciate the crisp numbers, super helpful. Good insight on the distance from the cup to transfer port, hadn't considered that but obvious in in hindsight.

Like I mentioned, I tried one, they sucked and then tried another. But the second round was perfect which is why I try to keep mentioning g which ones did work so that if anyone does it, maybe they can start with those orange ones and have it work the first time.

I think I'm going to flare some lines / cap the end and use them instead of crappy plastic parts.

 

[PDQ]

Can you use the plugs with the MC still mounted in the truck and "bench bleed" it via the pedal? I am having a fit bleeding the brakes on another truck and want to bench bleed the new master once more but would rather not pull it out of the truck again.

 

[DirtDonk]

Yes. You can do any of this stuff on the truck.
The whole point of “bench bleeding“ is to basically test the master cylinder before it’s installed so you don’t have to find problems after the fact and remove it.
It’s also so you can do it with one person, rather than tying up a second mechanic or helper.
You’ve already installed it, so go ahead and “bench bleed“ it on the vehicle.

 

[ksagis]

Can you use the plugs with the MC still mounted in the truck and "bench bleed" it via the pedal? I am having a fit bleeding the brakes on another truck and want to bench bleed the new master once more but would rather not pull it out of the truck again.

No reason why not. Couple of things to be careful of: 1) don't apply too much force with your foot since the plugs might pop out, and 2) fluid does splash around a bit so be careful of that.

 

[ksagis]

For the last 10 years or so for bench bleeding a MC the syringe and modified plug method is my go to, but all methods mentioned can work...... Mount MC in vise with a catch pan fill MC with clean fluid, Fill the syringe from one of the res. remove any air from syringe inject slowly in the port .{ you will see the air turn to a steady stream of brake fluid in MC res}once or twice for each port is all it takes ,,, then I plug both ports with my thumb and middle finger and cycle the piston ,usually its good to go but if the piston moves more then a 1/16 then I let some fluid escape past my fingers and reseal ports while releasing the piston,95% this all it takes......My experience with the tube method and the janky plastic fittings is the fitting can sometimes allow air to be sucked in and you sit there cycling the piston and see air bubbles indefinitely .. ..I have the vacuum bleeder mentioned they work but with caution, they can actually pull air into the system { usually at the wheel cylinders} , so if using to exchange fluid and then fallow with manual bleeding maybe necessary.. I'll add that if bench bleeding a new MC cylinder and you can't get the results on the bench .. It can and does happen that you just got a defective MC ..

Wanted to ask about the above, I think what you are saying is to not depress the bore of the MC, and use the syringe to "push" fluid into the MC through the port, and backflow into the reservoir. Do I have that right?

And then after doing that a few times, you press on the MC bore and if you have too much travel - before you release the MC bore, you let some fluid out of the ports, and then release the MC bore?

I'm assuming the intent of the letting the fluid out with your fingers is to also release the bubbles that are causing the excessive travel?

 

[EPB72]

Wanted to ask about the above, I think what you are saying is to not depress the bore of the MC, and use the syringe to "push" fluid into the MC through the port, and backflow into the reservoir. Do I have that right?

And then after doing that a few times, you press on the MC bore and if you have too much travel - before you release the MC bore, you let some fluid out of the ports, and then release the MC bore?

I'm assuming the intent of the letting the fluid out with your fingers is to also release the bubbles that are causing the excessive travel?

Correct

 

[Apogee]

You can purchase metal inverted flare plugs in most of the common SAE sizes, and a lot of auto parts stores will typically have them in their bulk brake bins. Most are zinc plated steel, however I've seen them in brass and stainless steel as well, noting that they should be stock items at industrial hydraulic shops as well, though often they only have the bigger ones. I don't have part numbers for 9/16-18 plugs, but I've used master cylinder adapter fittings to reduce the port size to something I do have a plug for, and that seems to work out pretty well in my experience.

FITTING, PLUG, 3/8-24 MIF (WH #131x3, Dorman #490-490, Edelmann #120300)
FITTING, PLUG, 7/16-24 MIF (WH #131x4, Dorman #490-491, Edelmann #)
FITTING, PLUG, 1/2-20 MIF (WH #131x5, Dorman #490-492, Edelmann #)

Tobin