Brake Problem

[september2012]

I have talked to you before about my smog problem. I think that is now fixed and I sent you an update over a year ago, however I have a new problem that is puzzling me.
 
I normally keep my '79 MGB at a storage location (residential garage) because I don't have room for all of my cars.  When I drove it there about 5 months ago the brakes were working fine, or so I thought.  Today I went to pick it up and the brake pedal went to the floor.  I pumped but nothing. I backed it into the street and sure enough there was no brake fluid in the master cylinder.  I drove my family car home and picked up some DOT 3 fluid and returned.  After filling the master cylinder, I pumped and pumped and I still had no brakes. I carefully drove the car home a few miles (ready to use the emergency if I needed) and it is now in my garage.  
 
I am puzzled about the vanishing brake fluid.  I would also think that I would have brake pedal once the master cylinder is filled, but I don't.  Could this be a power assist problem?  I saw no fluid on the garage where it was stored.  I can't believe that bleeding the brakes would cure this problem, but I will try it in the morning.
 
Looking for your thoughts when you have time. Thanks.
 
Lyle Abel

[Art]

Hi Lyle,
 
Glad to hear the smog problems are behind you, but this I'm sure is a puzzler.
 
Don't laugh, but they may actually be related.  Just a quick question - did the car smoke at all when you started it?
 
Unless you see a large oil puddle somewhere indicating a line or wheel cylinder/caliper leaking - which should only drain one of the master cylinder piston chambers, but would be an obvious source of the loss - check the power brake booster.
 
Often, as the seals wear and unltimately fail, the oil leaks through the back of the master cylinder and goes into the body of the power brake booster.  This is both aided by the fact that the chamber is under negative pressure (vacuum operated), which helps both draw the oil into the boostr and ultimately, through the vacuum tube to the intake manifold and into the engine, making it appear like you are burining oil (and giving a phantom hydrocarbon source, throwing the emissions out-of-wack).
 
This deterioration is often over time, so you may have seen small loses of brake fluid you passed off to brake wear.  This was not a catastrophic failure, probably draining slowly through the worn seals while in storage.
 
Pumping the brakes while driving did put air into the system, so just topping it off will not restore the brakes without bleeding everything now, but I'd check the back of the master cylinder for leakage (easy enough to remove the 2 bolts and push forward - no other connection to the booster) and see if it needs replacement.
 
I hope that helps you.  Let me know what you find.
 
Safety Fast!
 
Art

[september2012]

Thanks for the response Art and by the way thank you for all of the time that you spend writing technical assistance for the club.  
 
I did bleed the brakes the next morning.  When my wife would press on the brake pedal, and with the bleed screw open only a trickle of fluid would come out.  this is with the reservoir full of fluid.  All 4 brakes did the same.  I remember a stream of fluid when I did this before.
 
I have taken the car to a mechanic (questionable ability) and he is ordering a new master cylinder.  I have asked him where the fluid went. He doesn't know at this point.  Sometimes I think that I am better off doing all of this myself. Getting too old to crawl around under cars though.
 
If the fluid is draining into the booster, how do I get the fluid out of there, or will the brakes push the fluid out when the new master cylinder is installed?
 
Thanks again Art. Hope all is going well with you.  I will let you know how this works out.
 
Lyle Abel

[Art]

Hi Lyle,

Likely it is the master cylinder, so that's a good move.  The lack of flow bleeding may be due to internal bypass in the master (the fluid just sloshing around the pistons because of bad seals instead of generating pressure) or draining out the back seal to the booster.

It could also be a blockage, like at the pressure-cross valve on the firewall under the master.  You should check the failure switch for leakage as well as that can drain the system.  Still, you would see a puddle somewhere outside the car.

If the oil is in the booster, it will have to be drained.  Possibly replaced, as the fluid will destroy its seals.  Most times, it is carried off to the engine through the vacuum tube and burned off.  Small amounts would not even be seen as smoke.

All needs to be check.  At near 35, a car's life is like dog years and some of these components are at 2-3 times their life expectancy.

Good luck and keep me posted.

Best always,

Art Isaacs