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Brake master cylinder-dual line problem

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John75

John75

Freshman Member
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I replaced my brake master cylinder-dual line today in my 75 spitfire with a new one, but I'm not getting any brake fluid when I try bleeding the rear brakes. Front brakes bleed good. I disconected the rear brake line at the master cylinder, and there isn't any fluid coming out of the master cylinder when the brake is depressed. Fluid is topped off. Am I missing something here? /ubbthreads/images/graemlins/confused.gif Is it possible that the new master cylinder is bad. Should the same amount of brake fluid come out of both the front and rear brake line connections at the master cylinder. If anyone has experience with this, I could use some guidance.

John
Newbie with a 75 Spitfire
 
ISTR that the Spitfire uses a PDWA like the TR6. Is your brake warning light on? If so, look at the two lines that run from the MC to the PDWA that sits below the MC. Remove the warning light switch on the PDWA and insert an awl into the light switch hole to center the valve inside. If it has moved to one side during your MC work, it could prevent fluid from moving to one axle. Hope that this helps, I own neither a Spitfire or TR6. PeterK
 
I'll make sure the valve is centered. The light probably isn't working. This is a new Spit to me that's in need of many repairs after being neglected for several years. The brake fluid isn't even making it into the rear brake pipe union from the MC, just the front brake pipe union. I was thinking of taking it back out and trying to bench blead it to verify that the rear outlet on the MC wasn't releasing fluid from it.

This forum has a wealth of knowledge. Thanks for sharing.
John
75 spitfire
 
So how is the master cylinder project going? Hope Sunday was a better day for the project.

Don
 
Terrible Sunday trying to figure this out /ubbthreads/images/graemlins/frown.gif I was just checking the forum again to see if anyone else had ideas. I tried bench bleading the MC off the car, but still couldn't get any fluid to come out of the rear MC outlet. Front outlet is still fine. I can feel pressure when I cover the rear outlet with my finger, but no brake fluid comes out. This problem is looking more like a problem with the new MC or my rather limited knowledge of how a dual line brake mster cylinder works. Advice is appreciated. The weather has been great here for driving and I've been in the garage the past two days. /ubbthreads/images/graemlins/cryin.gif

John
75 Spitfire
 
I think it's time for another visit to Victoria British.... Sounds like a faulty part if you can't bench test it. Normally thats the first thing you do with a new master cylinder to make sure there is no air trapped somewhere inside.

See you at the KC All British Show.....I can't wait.

Don
 
[ QUOTE ]
I tried bench bleeding the MC off the car, but still couldn't get any fluid to come out of the rear MC outlet. Front outlet is still fine. I can feel pressure when I cover the rear outlet with my finger, but no brake fluid comes out. This problem is looking more like a problem with the new MC or my rather limited knowledge of how a dual line brake mster cylinder works.
John
75 Spitfire

[/ QUOTE ]
Hi John,
The piston for the back outlet relies primarily on hydraulic pressure from the front piston to actuate it. When bleeding, the front port has to be blocked off, before the rear piston will operate correctly. It's best to install two separate temporary plugs, bleed the two parts of the cylinder separately, & leave the plugs in until you are ready to connect the lines.

In operation, the front piston builds hydraulic pressure, usually on the front brake system, AND on the back piston. The back piston in turn applies pressure to the rear brakes. There is a short mechanical safety push rod between the two pistons that will apply the rear brakes if pressure fails to build in the front. This usually occurs only when the pedal is near the floor.

Some master cylinders have the front & back positions/functions reversed from this description.
D
 
Hi Dave,
Thanks for the information on how the dual line MC works. Your explanation is exactly how mine is set up. Front port front brakes, rear port rear breaks. I did plug the front port when I bench bled the MC with a correctly threaded bolt, and made sure there was no air leaking by wrapping the bolt threads with teflon tape. I still couldn't get any fluid out of the rear port. I started with light pressure and then made sure the piston went all the way in and stopped. Any other suggestions are appreciated. I don't want to look like a fool returning the MC if it's really operator error. /ubbthreads/images/graemlins/blush.gif

John
75 Spitfire
 
I think I said, or meant to say that the front port (front of the car as installed) goes to the rear brakes. Just reverse front & rear in my discussion & pic, if yours is different.

The only thing I can think of is that the front piston (toward the front of the car as installed - rear brake) is not retracting far enough to uncover the reservoir port to allow fluid into it. This could be caused by compressible air being trapped between the front & rear pistons. ie, the rear piston (toward the rear of the car as installed - front brake) is not is not fully bled. This would hold the front piston slightly forward so that the reservoir port is not exposed to let fluid in. Make sure that the temporary plugs are sealed as appropriate. It's possible but not likely, that the MC was assembled wrong.

You should be able to look into the reservoir & see the small ports at the bottom & whether the pistons are retracting far enough to uncover the ports & let fluid in.
Maybe the attached pic will help.
D
 

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  • 131463-DualMC.jpg
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Dave,
Thanks for the drawing. I'm including a picture of my old disassembled MC with labels. My MC doesn't have ports that I can see in. It has one tipping valve that feeds both brake lines. I've also tried a Mityvac vacum bleeder while on the bench with no luck getting fluid out of the rear brake port. I'll try carefully bench bleading it again on Monday making sure to seal off the bolts before giving up and taking it back. Once again the picture is of my old MC not the new one I'm trying to bench bleed. New one looks exactly like this only cleaner.

John
75 Spitfire
 

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  • 131485-75SpitfireMasterCylinder.jpg
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Hi John,
I see what you mean about the ports not being visible. This MC is somewhat more complex than others.

I can send you a writeup with pics, of the MC, & how it is supposed to work, in PDF format. You can send your email address to me here, if you wish. rusd@velocitus.net
D
 
Dave,
Thanks for the PDF file. It really helped me confirm to take the MC back to Victoria British (lucky I live 5 miles away from VB) and exchange it. Did that and the new one bench bled both ports with no trouble. I put it on and now I'm experiencing lots of air in the rear lines and have been unable to bleed the rear line. Even after 30 minutes of using the Mity-Vac and traditional 2 man brake bleeding technique. I've bled a full bottle of clean brake fluid out of the rear lines, so I know the new MC is delivering the fluid. I think I've got the front line bled. I'm starting to wonder if I have a brake line vacum leak leading to the rear breaks. There's no fluid dripping along the path, but if the hole is on top of the line I'm wondering if it wouldn't leak without that line being full. I'm going to tear down my rear brake drums on Tuesday (have to work tonight) and see if the Rear Wheel cylinders are bad causing air to enter the brake lines.

Thanks again,
John
75 Spitfire
 
Hi John,
Glad you are making progress.

First question, how do the brakes feel - work?

Second, it's nearly impossible to get an air free indication with a vacuum bleeder unless the bleeder screw threads are sealed with teflon tape. Even then it is sometimes difficult if there is any vacuum leak around the bleed screw threads. Air may also leak back around the wheel cylinder seals since they are pretty much a one way seal.

Third, there is sometimes, a residual pressure valve in the line somewhere to keep a positive pressure, maybe 5-10 psi, on the rear brakes at all times. If it is there, it would likely be in the MC outlet. I think it is called a spacer/check valve. This is to prevent air from being drawn back into the system via the wheel cylinder piston seals. There could a similar arrangement to maintain about 2 psi residual pressure on disc brakes.
D
 
Dave,
I definately feel like I'm making progress. The brakes are firmer, but not as hard as I think they should be. I remember reading about using vacum's to bleed brakes and that it doesn't remove all the air. That's why I also did it the old fashioned way (with two people) after using the vacum. I did pick up some new bleader valves that I'm going to install on Tuesday and then bleed the brakes again. If they feel solid after that I'll put the rear tires on and roll it down the drivway to see how she stops. /ubbthreads/images/graemlins/eek.gif

If they don't feel right, I'll pull the brake drums and see what's cooking with the rear wheel cylinders. If they're old, they might also be creating a slow vacum leak.

I didn't see anything resembling a spacer check valve on or near the MC, and I don't see one mentioned in the Haynes book or parts catalogs. If it exists it may be built into the MC where I can't see. At least I'm glad to have figured out the MC problem. /ubbthreads/images/graemlins/smile.gif

Thanks,
John
75 Spitfire in need of tender loving care.
 
Continuing to have problems with bleeding the brakes. They feel squishy and eventually the pedal goes to the floor every time. If I pump the brakes up there is enough force to then engage the brakes, but then they fail as pressure is kept on the peddle. I went through another bottle of break fluid bleeding the lines and the brake never stays firm.

Disconected the brake lines to the MC tonight, and tried bench bleeding it again. This time I left it in the car, using bolts and teflon tape to seal each port in turn. Still squishy.

I even tried plugging both ports and stepping on the brake peddle and got the same squishy results. With both outlet ports on the MC plugged, shouldn't the the brake peddle stay firm and not allow the pedal to travel to the floor? /ubbthreads/images/graemlins/confused.gif

John
75 Spitfire
 
[ QUOTE ]

I even tried plugging both ports and stepping on the brake peddle and got the same squishy results. With both outlet ports on the MC plugged, shouldn't the the brake peddle stay firm and not allow the pedal to travel to the floor?
John
75 Spitfire

[/ QUOTE ]
Yes, it certainly should. Even with some air in it it it might spring a bit & then come up firm once the "spring" was taken up. Hard to believe that you could get two in a row that were bad, but maybe they were all done at the same time & place.
D
 
In closing this thread:
the problem was two bad MC's in a row. I had my local mechanic try bleeding the breakes and he immediately said the second MC was bad. Got another new one and 20 minutes later had the breaks bled and was rolling and stopping /ubbthreads/images/graemlins/cool.gif down the road. Just in time to attend the All British Car Show in Kansas City this weekend. Thanks especially to Dave for helping me through this, I've learned a lot.

John
75 Spitfire /ubbthreads/images/graemlins/driving.gif
 
John,

Glad you finally got the master cylinder problem fixed. Two bad master cylinders in a row is unacceptable. I had problems with silent-bloc bushings from Victoria British. I'm not that big of a fan of them, even with living so close. I sure wish the Roadster Factory was near us.

I've been busy cleaning up the TR3B for the show, see you there.

Don
 
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