healey

Looking for some advice from the 928 experts on the forum as to loss of brake fluid in my '79 OB. I store the car during the winter and today went out to start the engine to get the fluids warmed up and move the car a bit to avoid flat spots in the tires. I notice the brake fluid warning light flashing and found the reservoir empty. I checked under the car and could find no sign of any fluid on the floor. I added about 5-6 oz of brake fluid and was able to pump the brakes and get a hard peddle. My questions is where could the missing brake fluid have gone? I changed the blue hose about 6 years ago. I was not able to check for leakage around brake Master cylinder but assumed if it was leaking that fluid would show up on the floor. Any suggestions or insight would be appreciated. Thanks

Mrmerlin

if you will post a clear picture of the brake booster and master cylinder ,
and also a picture of the clutch and brake MCs inside the cabin it would help with an answer

Rob Edwards

If not on the floor, either into the brake booster, or onto the carpet in the footwell, as Stan is suggesting.

Billu

dr bob

You can use a piece of hose or wire through the booster can hole after removing the check valve from the grommet. Fluid may gather in the bottom of the can, and if so you'll find the end of your test probe wet with fluid. You may also find traces of fluid in the vacuum hose from the booster.

Mrmerlin

Hold on folks,
if their is fluid in the booster then the booster and the master cylinder should both be replaced.
A new ATE booster is 275.00 USD ,
the ATE MC is about 265.00,
A set of 4 stainless flex lines about 88.00,
and a liter of ATE gold fluid is about 19.00.
this pretty much makes the brakes great again for another 30 years
Seriously you dont want to take short cuts on brake systems

Wisconsin Joe

The old "Car Talk" show on NPR was lots of fun, and very informative, from a basic knowledge point of view.

The guys had 3 levels of car problems:

1 - Will cost you more money if you ignore it.
2 - Will leave you stranded and cost you more money if you ignore it.
3 - Will KILL YOU if you ignore it.

Brakes (steering, fuel leaks and a few other things) were a "Class 3" problem.

docmirror

Technically speaking the booster is not a hydraulic system, it is a pneumatic system which aids the physical pressure applied to the pedal. If the booster is 'leaking' it will be leaking air from the vacuum supply. While a failing booster will cause heavy pedal pressure, and some engine roughness/hunting, it is not an issue with the fluid loss except it may be the location that the fluid is getting trapped. Using a vac tester, it is possible to check the booster for air leaks. Replacing it to solve a fluid loss would not be indicated.

More likely is the brake MC. If no stains on the inside of the footwell, then it's going into the booster chamber, and should be drained when the MC is replaced. Stan mentions, new flex hoses at the same time, and that will keep the brakes in good shape for a while. Unless one of the caliper pistons is getting corroded, which can only be investigated with a full caliper disassembly.

Mrmerlin

a leaking brake MC will fill the booster,
once the booster is half full then it will leak into the cabin this will happen after you have filled the MC tank a few times.

That said, once brake fluid is leaking into the booster I strongly suggest that the booster also be replaced.

NOTE since brake fluid is hygroscopic it will absorb water this will then fill the inside of the booster and cause corrosion,
the brake fluid may also damage the rubber diaphragm and then leaks will occur.
Trust me its a lot easier to replace a old booster with the master cylinder removed

healey

Thanks everyone for their insights. I did not notice any leaking fluid on floor or carpet under the pedal but will check more carefully this weekend and then look at the booster as so many mentioned and if there is brake fluid in the booster I will look at replacing as Mrmerlin suggests. I did replace the MC a few years ago so hopefully it is OK. Thanks again for everyone's input.

healey

I finally pulled the master cylinder and brake booster. What a chore. I did find some pointers on Rennlist which helped but had to also remove the 2 brake pressure regulators to get the booster out. I found brake fluid leaking from the MC into the booster. Planning to replace the MC but the brake booster is a different story as I have an early '79 with a 9" booster which is NLA. I drained the brake fluid from it. Any idea how I can test the vacuum seal in the booster? Another option is to have the booster rebuilt. I read somewhere on Rennlist that White Post in VA or Karp's in CA rebuilds these. So, what is the consensus on what to do with the booster? Re-install or rebuild? Not sure of the cost of the latter but would rather not incur that expense if the booster is basically OK.
Thanks in advance for any suggestions.

Mrmerlin

If you call Roger he will send you the updated 10 inch ATE booster also buy the updated clevis so it will reach the pedal.
NOTE the 9 inch booster is not made anymore/available and was superseded, the new 10 inch part offers more force so the brakes work better.
NOTE the 10 inch booster is also made differently so its easier to install and remove.
NOTE Use ATE Fluid Roger has it.
Always fit a new ATE master cylinder when refitting a new booster.
Use DC111 on the MC fluid inlet, and the tank grommets and the MC base seal.
Be careful not to scratch the sealing area of the MC rubber to the booster face,
make a cover from the side of an oil can to protect the face while you install it.
NOTE reread post six for current prices,
if you follow my instructions your brakes will work for a very long time without any more rework

jpitman2

My reservoir was empty when I picked up the car! Fluid was all over the inner fender, and had leaked because the reservoir grommets had turned to liquorice . About 10 years later the new ones had failed also. Cheapest from an MB dealer.
jp 83 Euro S AT 57k

Mrmerlin

JP the reason the grommets leaked was because the MC inlet bores were corroding,
this changes the sealing surface for the grommets.

NOTE once you have a leak at the grommets its a very safe bet to install a new ATE master cylinder and use DC111 on the grommets to keep out the water.
installing new grommets on a uneven corroded surface can result in leaks later BTDT.

ROG100

This is a 79 car with a 9" Booster which is NLA. A 10" booster will not fit without serious readjustment of the fender/engine bay with a large hammer.
My advice is to have the 9" booster rebuilt and be done, after all it is 40 years old.