bad MC and testing

1930artdeco

1930artdeco

Well-known member
Hello,

I may have a bad MC (brand new) I can't seem to get it to build pressure and I have no air coming out of it when I bleed the brakes. What is the best way to test to see if it has a leak?

Thanks,

Mike
 
If fluid flows out during the bleeding process, it should push the air out as well, at least till it's bled. If it's pushing fluid but won't build pressure then there may be a problem with the master or any one of the wheel cylinders or brake mechanisms. A bad flex hose can balloon during brake action but they will generally fail before that happens. Wheel cylinders with a problem should be gushing fluid out unless there is too much shoe clearance with the drums.

Some folks use a pressure bleeder to force fluid back to the master reservoir. If it's completely purged of air and still won't build pressure then there would be a bad rubber cup in there or something that it bypassing fluid.
 
That is my guess Rotorwrench. I don't see any fluid coming out of the cylinders but I will bleed it again a few times to verify. But how do I test to see if the MC is bad?
 
After a good bleeding, when you get a good pedal, stand on it for a full minute. Does it go down? If so, check out another MC.
 
If a MC is suspect, you simply plug the outlet(s) with block-off plugs. If the pedal is hard, the MC is good. If soft or no pedal, either bench bleed again or replace.

Actuation rod is correct type and adjusted correctly? Shoes have to be adjusted correctly.

Booster used?
 
It has the original rod, and is a brand new MC. Shoes adjusted correctly, but did not bench bleed. Will check this weekend, I hope.
 
If the master is firewall mounted with swing pedals, they bleed easy. If it's under the floor, not so much. High mounted will almost bleed themselves if the bleeder nipples are opened up in proper sequence and just let the fluid flow a bit by gravity for each one starting closest then on to farthest away. Low mounted usually required a hand operated vacuum pump and a jar to catch the fluid. I put teflon tape on the nipple threads for the loosey goosey ones. I've been doing them by myself for years and have had good results.

As was mentioned, the master can be blocked to pressure test but a person needs the correct fittings to do that. They have banjo fittings on some and the tubes can be the inverted flair types.
 
I use a Mityvac to bleed brake lines after installing a new MC. Once all connections are made and tight, fill the MC with brake fluid, then open the farthest wheel cylinder bleed screw and put a rubber (or clear) hose on the tip of the bleeder screw so you don't lose any fluid. Run that hose to your MityVac and sqeeze the trigger. Watch fluid and air bubbles fill into the plastic reservoir on the MityVac until nothing but fluid goes into the reservoir. Go up and fill the MC again. Close that bleeder screw then go to the next farthest (from the MC) wheel cylinder and repeat the process. When all have been bled, you should have a hard pedal.
If the pedal is soft and goes to the floor, most likely the MC has a bad piston cup inside it.
Make sure brake shoes are properly adjusted before condemning MC.
 
Last edited:
Thanks Dave, Do you use the hand or shop air version? I am debating which one to buy as I only have one vintage car with hydraulic brakes.
 
... :confused: ...​

'ya know ... this is really getting scary ...

Little wonder why the FEDS are wanting to disallow owners from working on their cars.

The only consideration in bench bleeding is not to extend the piston full bore as it may cause damage to the seal.

You cannot bleed on the car unless the MC is perfectly level.

Here is the safest way to bleed (without expensive pressure bleeding equipment) -

https://techtalk.mpbrakes.com/bleeding-a-master-cylinder

And taking advice from The H.A.M.Bone is dangerous. They still pass OLD WIVES TALES over there that were proven incorrect years ago. I had to get away from there.

One can only wonder why no one is asking why no whale oil in brake fluid ...
 
Last edited:
1930artdeco said:
Thanks Dave, Do you use the hand or shop air version? I am debating which one to buy as I only have one vintage car with hydraulic brakes.
Click to expand...

Doesn't matter how old your car is, you can use the hand operated Mity-Vac for wheel cylinders of all years, just slightly different hose sizes, but adaptors come with the kit. I've used the hand-operated model for many years on both new and old cars. Best thing about them is it is easy to bleed brakes and only takes one person. That is a must for me since I have never had any help here.
Mity-Vac was in-expensive too back in the day when I bought mine. Don't know how much they've gone up by now.
 
Just curious about bench bleeding. If you fill and bleed the MC on the bench and then install it into the car you have to hook up open lines to it. Those open lines have air in them, isn't that defeating the purpose of the bench bleed?
 
I was fleet mechanic for 30-years and replaced hundreds of them. You must bench bleed them. We would clamp them in a vise, stick our finger over the hole and push the piston in with a large punch. You finger will be pushed away with the pressure and then go back over the hole on the return stroke. You do that a few times until the fluid squirts all over you. Makes a big mess but its fast. Then bolt it on the car and leave the line loose and have someone "slowly" push on the pedal and then tighten the line when you get fluid. Then move to the furthest wheel cylinder from the master cylinder and bleed it first. Make sure the master cylinders' piston can return all the way back to the snap ring. Always start the brake line first and finger tighten it before you tighten the master cylinder bolts. It makes it a lot easier to get those lines started, same with wheel cylinders start the lines before you put the bolts in. The lines need to be tight but if you tighten them too much you will split the flared end of the tubing and have a leak. Road test the vehicle and then check for leaks.

Some people use plugs which forces the air back up through the return port in the reservoir. Others use brake lines that are bend around from the port and go into the reservoir, you keep pumping until you don't see air bubbles in the reservoir. If you don't get that air out of the master cylinder first, you won't be able to bleed the system. If the master cylinder is higher than the wheel cylinder you can gravity bleed it the wheel cylinders. Leave master cylinder cap off, open the wheel cylinders one at a time and when its dripping shut the bleeder. I used to just let them gravity bleed on the hoist with a trash can under them while I was working on something else. Keep an eye on the master cylinder level, If you run it empty your starting all over again.
 
Last edited:
1930artdeco said:
Just curious about bench bleeding. If you fill and bleed the MC on the bench and then install it into the car you have to hook up open lines to it. Those open lines have air in them, isn't that defeating the purpose of the bench bleed?
Click to expand...

When you bench bleed you have a little air left at the end of the master cylinder where the line goes. You leave the line loose and have somebody push down on the pedal which forces that last little bit of air out of the master cylinder and then you tighten the line when the fluid starts to come out. From then on fluid will be forced down the lines and any air will be ahead of the fluid and pushed out. An air bubble cannot go anywhere but down and out. Do not rapidly pump the pedal like your see people do. It can bust up a large bubble into a million little ones. Open the bleeder, push down slow and easy, close the bleeder, let the pedal return and repeat until you get a nice stream of fluid. My dad always put a rubber hose over the bleeder into a jar so he didn't make mess. At work we just let it squirt out on the floor. It did a good job of cleaning the concrete when we mopped it up. Remember that it will take paint off of your restored cars so wipe it off immediately and rinse with water.
 
KULTULZ said:
... :confused: ...​
And taking advice from The H.A.M.Bone is dangerous. They still pass OLD WIVES TALES over there that were proven incorrect years ago. I had to get away from there.

One can only wonder why no one is asking why no whale oil in brake fluid ...
Click to expand...

FYI, Ryan owns/runs this site as well as the HAMB. There's quite a few HAMBers on this site also. Just sayin'.
 
KULTULZ said:
... :confused: ...​

And taking advice from The H.A.M.Bone is dangerous. They still pass OLD WIVES TALES over there that were proven incorrect years ago. I had to get away from there.

One can only wonder why no one is asking why no whale oil in brake fluid ...
Click to expand...

evintho said:
FYI, Ryan owns/runs this site as well as the HAMB. There's quite a few HAMBers on this site also. Just sayin'.
Click to expand...

I know that. I also know that what is posted over there about hydraulic brakes on a continual basis is scary, the same as here.

What is so hard to understand the MC has to be bench bled before installing it on the car?

Why does this bother you personally?

Just sayin' ... :cool:
 
I might as well step on a few other toes while retaining water -

Just curious about bench bleeding. If you fill and bleed the MC on the bench and then install it into the car you have to hook up open lines to it. Those open lines have air in them, isn't that defeating the purpose of the bench bleed?
Click to expand...

No, it is not defeating the bench bleed.

When you bench bleed you have a little air left at the end of the master cylinder where the line goes. You leave the line loose and have somebody push down on the pedal which forces that last little bit of air out of the master cylinder and then you tighten the line when the fluid starts to come out. From then on fluid will be forced down the lines and any air will be ahead of the fluid and pushed out. An air bubble cannot go anywhere but down and out.
Click to expand...

If the MC is bench bled correctly, there will be no air in the MC.

The problem(s) occurs when the lines are attached to the MC and the fluid beyond the check valve(s) runs out while tightening the line fittings and there is resulting air introduced into the lines.

If you are really lucky, having someone slowly push the brake pedal slightly will force the air out while tightening the fitting(s).

Now in my world this is called taking a chance. The complete system needs to be bled at that point especially if the original MC was defective in such a way as it introduced air into the system. This will also flush the system of old fluid and possibly any contaminates in the system.

Air will always rise in a closed system. One good reason for reverse bleeding.
 
Did you bench bleed the unit before installation?

My Dad always likes to jump in an immediately start pumping the pedal like a mad man. Ticks me totally off!

My Vacuum gun works ok, except on under floor units. In that case the residual valves do not play well with the gun for some reason.
 
You must log in or register to reply here.
Back
Top