Bleeding that dual master system

Academic question.

Many of us have installed the dual master brake system. Today I was thinking about my installation and a question came up. With the two independent systems, how do you know both systems are bled properly? Could you have one system perfect and some air in the other and not know it because you won't have a mushy pedal?

I know when we bled mine the pedal got stiff after the rear was done but we still had to do the front. Same for a brake failure, other then noticing loss of braking, will you notice anything in the pedal movement?

Be thankful you got yours to bleed right. I tried to vette master cylinders and couldn't get either to bleed at the wheel cylinders. I gave up. I NEVER had this problem ever when replacing a master cylinder in any vehicle over the past 20 years.
I gave up...and reinstalled the stocker...

If you don't bench bleed the master cyl BEFORE you conect the lines you will have problems getting fluid to the wheel cylinders

Funny, but I can't bleed the brakes in my CJ to save my life right now, but had no problem when I installed the new dual master cylinder in the M715. I bled the m/c before installing it, then I just bled the RR, LR then RF and LF until clean fluid was flowing and called it good.

I've asked this before but don't remember the answer. What exactly is bench bleeding? I have my m/c in a vice and put fluid in it and pump it a few times to get all the air out or what?
I mean, why can't that process be done while its in the truck???
Coincidentally, I'm doing mine right now too.
Also, I used 3/16th line right from the m/c to the system, the front's are all 3/16th but the rear has the 1/4" like the way it came stock. The guy at NAPA said it shouldn't make any difference but I guess I'll find out.
Last night I told my wife it's time to do brakes again she just sighed and rolled her eyes. I had trouble last time untill I bought one of those suction bleeders but after I learned how to use it,it bled the system great.

It can be done installed. You just disconnect your line/s and run a spare brake line in a loop, from the outlet and back into the MC reservoir. That's how I've done it in the past, because I didn't see the sense in removing the MC from the vehicle each time the reservoir dried up. :)

I think the reference to "bench" is due to the process of bench bleeding typically being required only when replacing a MC, so the part is already out of the vehicle anyway and it's an easier process on a shop bench than under a hood.

In regards to the original post, I can only make assumptions based on the loss of one side of the dual system. I assume that the pedal would feel some loss of resistance, as it pushes the MC piston through the fluid in both reservoirs, but would also retain partial resistance that would enable brake engagement on the good side. My assumption would also suggest that, since both reservoirs rely on the single piston inside the MC, that pedal pressure would indicate whether both lines are cleared of air or not.

Quote:

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Originally Posted by fng

I mean, why can't that process be done while its in the truck???

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It can be. I did mine that way. The MC came with a kit of plastic hose adaptors, tubing and a little clamp, all for bleeding. I put the bleed kit on, mounted it in the truck, filled it up and in less then 5 mins of slow pumping, it was done. Then, I removed the kit, hooked up the steel lines and bled the entire system. A big plus is no dripping MC to install if done this way.

One easy way to see if both independent circuits are working is the brake lights. They will only activate if your front circuit has pressure. So, if the truck stops and you got no lights, the fronts aren't helping any. Agengr has that exact problem with his.

Bleeding the master before the lines are hooked up in any way is basically what "bench bleeding" means. Holding your fingers over the outlets while somebody else pushes the pedal real slow will work as well. Keep going until your fingers are pushed off or bubbles stop coming up inside from the pistons.

Once the master is bled, manual bleeding, hand pump vacuum, automatic vacuum or pressure bleeding all work. Actually, pressure bleeding works with a non bled master. I can't find an adaptor to fit the huge 'vette master however. I gave thought to cutting up my old one and making it into one, but just put Colton to work pumping instead.

I like the hand pump vacuum type when working by myself on just about any kind of vehicle. Starting with a dry system however will wear your arms out getting that passenger side rear to ever get fluid.

That is when I hook my a/c vacuum pump up and plug it in. 10 seconds later fluid at the rear. Be carefull doing this since it will suck the master dry super fast.

I have a air powered vacuum bleeder at work which is great for this kind of job. Pressure bleeding is best however when thinking of speed, fluid wasted and use on any newer ABS equiped vehicle. They can be made from garden sprayer tanks for about $30.00. Warthog has a great post about how he made his. Recovery 4x4 on SS made the same basic thing as well.

I would also suggest DOT 5 or rather "Brake Fluid, Silicone" if buying surplus. My 'vette master lasted a little more than 4 years before the bore rusted out with DOT 3 in it. It is flushed and filled with BFS now along with a new master. Spruce Mt surplus has gallons of BFS for $30.00 a gallon which came to $70 for 2 gallons delivered to my door a few weeks ago.