The Stovebolt Site Forums The Shop Area Driveline bleeding master cylinder

I have my master cylinder sitting in the frame of my 41, it's bolted in place but I've heard the talk of bench bleeding, is there any advantage of doing it on a bench or right where it sits. It's in a good spot, any reason not to do it right where it sits? I have all the rest done, all new tubing and wheel cylinders. Dave

I'm not sure about the ease of the operation but I do know that you don't want the fluid to touch any painted surface. Also it's very moisture sensitive. Brake fluid is a mess. I use a pressure system and just flush the whole system. I use a old water bottle with a piece of vacuum line punched through the cap. The bottle hangs from the calipers by a coat hanger I rigged. Hook it up and flush until the bubbles stop and clean fluid enters the bottle.

If your lines are new and have not been filled with fluid then "bench bleeding" is irrelevant. The amount of air in the MC is nothing compared to the air in the lines. If the lines have been filled (at some point) with fluid then try this: Connect the lines to the MC. Crack open the lines just like a bleeder screw while pressing down on the brake peddle. Close the connection before releasing the peddle. This will force most of the air out of the connection instead of forcing it all the way through the lines. You may still need to bleed the lines if the peddle is spongy.

The idea is that you want to get as much air out of the master cylinder as possible before it is hooked up to the rest of the brake system. That does a few things, one, it ensures that you don't push any more air into the system than is already present. Two, it also ensures that you get all of the air out the ports in the master cylinder itself, which can be very tricky to do. This also has the side benefit of preventing cavitation of the fluid when bleeding the rest of the system, especially if you're bleeding it manually.
That said, not all master cylinders require bench bleeding, a good example would the single circuit master cylinders found on pre-'59 trucks.
You don't mention in your post what type master cylinder you are using (stock? dual circuit?) but if it is one that should be bench bled, you should be able to do it in the frame. Bleed it in the same manner as on the bench, just do it before you hook the rest of the lines up.
A little tip if you should ever have to replace one where the rest of the system hasn't been touched to make bleeding easier/quicker- After bench bleeding and installing the master cylinder, install the lines leaving the fittings slightly loose. Then have a helper slowly push down the pedal while holding a rag around the fittings, then tighten them up while the helper holds the pedal down. This will push out any air that's in the upper parts of the lines, instead of pushing it further down. You will still want to go ahead and bleed the system to be 100% sure there's no air, and also to flush out any contaminants in the system.
BTW, I'm in agreement with Hollow65, pressure bleeding is the best way to flush the system out and bleed it at the same time.

All new everything,rebuilt original MC, lines,wheel cylinders. Dave

Should be able to bleed it fine on the truck then. If you have access to a pressure bleeder with the proper adapter for the master cylinder, that would be the easiest and fastest way to bleed the system. Make sure to use the proper sequence to get the best performance, bleeding the longest line first. For a '41, the sequence is LR, LF, RR, RF.

Thanks. Dave

Agreed, pressure bleeding is the way to go.

However, if you don't have access to a pressure bleeder setup, vacuum bleeding is second best. Hook the vacuum to a bleeder jar and starting at the furthest wheel, just suck the fluid through. Keep an eye on the fluid level in the MC. If it runs low and pulls air you are starting all over again and it's a lot easier to pull the vacuum on empty lines.

I've used pressure bleeding and vacuum bleeding and even gravity bleeding when filling completely dry systems.
The easiest way I found was to get a large syringe from the farm supply store and push a syringe or two full of fluid through each bleeder port(with all the others closed at the same time)toward the master cyl. This pushes the air in the natural direction it wants to go. Works especially well when using DOT 5 fluid which likes to trap air when agitated.

I would strongly advise against using DOT5 silicone fluid in any vehicle that did not use it as a factory fill. There are compatibility issues with the rubber components, as well as issues with any moisture that gets in the system settling to the bottom of the cylinder, causing rust pitting. Had that happen to me, will never use silicone fluid again in anything other than a vehicle that sues it as OEM.

The moisture issue is one of the reasons for using DOT 5. It doesn't attract it like DOT3 or 4. In vehicles that see very little use, like the ones we build, it will cause less problems in the long run.
These systems are completely new, lines, hoses, rubber parts, etc. and have never had any other fluid in them. Usually, they won't reccomment DOT 5 because if the whole system isn't replaced or throughly flushed you can have compatibility issues with old fluid.
When you're talking about a car with a $7 to $10k paint job, the DOT 5 is nice because it won't ruin the paint.

The problem is that the replacement rubber components that are available are not designed to be used with DOT5, they are made for use with DOT3 or 4 glycol based fluids. DOT5 can destroy rubber parts that ate not made specifically for use with that type fluid. Also, while DOT5 will not attract water, it also will not disperse it like glycol based fluids will. That tends to allow any fluid in the system (remember, these trucks have vented master cylinders, moisture will get into them) to settle at the lowest points. That will not only allow for rust pitting, but can also lead to spongy/no brake pedal under baking conditions that get it hot enough to biol the water in the system.
As mentioned, I bought into the whole "DOT5 is better for old vehicles" thing, and did run it in my truck, which had a complete brake rebuild, with all new lines, master cylinder, wheel cylinders, and hoses. Took DOT about a year to ruin the cups in the wheel cylinder and master cylinder. I contacted the manufacturer, they told me that none of their products were rated for use with silicone fluid. Got the same response from all of the other brake parts manufacturers except for EIS, who is no longer in business. I have not had a problem since I went back to glycol fluid.

I also bought the "DOT 5 is better for old vehicles" thing about 13 years ago when I was "restoring" my 1954 Suburban. I had no problem calling all brake parts manufacturers to confirm DOT 5 compatibility with all rubber parts. The only problem that I had was with a Jim Carter master cylinder that was defective.

I should probably bleed the brake system, but I have not bled it in over 9 years.

Nonetheless, I will not use DOT 5 again. I now see no benefits.

Well, we've been using DOT 5 for 20 years at my shop with no problems or complaints. All 3 of my Harleys came with DOT 5 from the factory more than 20 years ago and I've never had any brake problems with them that weren't self-induced. I'll stick with the DOT5 and the big syringe.

DOT 5 is factory fill on Harleys, as well as several other motorcycle brands. Their brake systems are designed to be used with it. The brake systems in passenger vehicles and trucks are not, unless you convert them with DOT5 compatible rubber components. Best bet is to stick with what the manufacturer designed the system to work with if you use oem replacment parts.

I installed a dual reservoir style MC, I did a bench bleed before installing. Once hooked up to the lines I found the vacuum method using a hand pump wasn't working for me. What solved my problem with a dry system was to attach a hose to the brake cylinder bleeder screw. It was long enough for me to rig it up so it was held higher than the MC. I filled it(using and eye dropper) sounds ridiculous. But each time it pumped air out it sucked fluid in. Before you know it all the air was out. After much time wasted pumping etc this is the way I would approach another situation of the same when starting with a dry system. that my 0.02

Check out this home made brake bleeder, been using a similar set up for years, makes the job enjoyable.
Jim