1970 GT 350 Brake Booster or master cylinder?

[Bill]

Drove my May of 1969 built, 1970 GT 350 today and a block from home the pedal got hard, not quite to floor, but barely able to slow to a stop. Thinking perhaps I might have inadvertently cooked the brakes driving through construction, let the car cool for for three hours and just went out to take a look. Fluids all flushed with fresh DOT 3 2 years ago, master cylinder is full, rear bowl is a bit murky, front is fairly clear, no drips, no leaks either under the car or at all four corners. It's been 20 years since I last had a brake issue on any of my mustangs, now wondering if the master cylinder went out or if the booster did. Feels like no rear brakes at all, with a good bit of pressure needed just to stop the car from rolling, no vacuum leaks detected.

Believing earlier that the master cylinder went out, I proactively ordered a new one from Dead Nuts this afternoon, but what happens if it's the booster?
Being a May of 69 build, it can't be a Midland, can it?
Who is rebuilding or offering a good reproduction of the later production brake boosters these days?

[Road Reptile]

Hi Bill,
The first people to contact are the folks that run Power Brake Booster in Washington
(503) 238-8882  They have a great reputation and should be able to help with your questions and the rebuilding/restoration of a Bendix crimped type booster.
Still a good idea to remove the wheels and inspect closely....the last 69 G.T.500 That I was involved with had both rear wheel cylinders seized up.Most likely from sitting. Good thing the new owner had it trailered home, as he was told-Fly in and drive it home....All the best from a salesman
Please keep us posted on what you find.
R.R.

[Bill Collins]

Bill,
It's the booster - I have experienced this multiple times over the years with cars I have bought/sold. Either the diaphragm failed due to being old rubber or there is a leak out the back of the master cylinder into the booster and the fluid attacks the diaphragm, although this is less likely with DOT3 fluid.

A leak would not be immediately evident as the fluid gets sucked into the engine and burned in the combustion process.  The suggestion to check the rear wheel cylinders is also good advice.

My recommendation is to have the booster rebuilt. You can check the back of the master cylinder for a leak while the unit is apart. Here is a good source for booster rebuilding, I have used them several times with good results:

https://www.powerbrakebooster.com/

[jimhyc]

Bill-
I was just going to post the exact same question.  Last July I bought a 69 GT500 that hadn't been driven much over the last 30 years. The brakes weren't great but worked decent. Over the winter I replaced the original rear hose, fronts were already replaced, wheel cylinders. Rebuilt the Master cylinder, proportioning valve (it was totally seized), caliper pins and boots. Full panic stomp on the pedal and it barely slows down. Pump the pedal a few times, start the car and the pedal goes down a bit like it should. I don't hear a hiss from the booster under the dash and with a vacuum gauge I don't see a leak.

[roddster]

  Seems to me that the slight change in the Fuel chemistry has had an effect on the booster diaphrams.  They don't seem to last as long if they haven't been rebuilt with new stuff.

[Shelbypat]

I have had same issue a few years ago and went with a Dewey brake Booster replacement.

[Bill]

Thanks guys

[Bob Gaines]

If your booster is nice on the outside (no rust pits) you may want to confirm that they are rebuilding what you send and not an exchange where you may get one of lesser quality back.

[Bill]

If your booster is nice on the outside (no rust pits) you may want to confirm that they are rebuilding what you send and not an exchange where you may get one of lesser quality back.

Thanks, I'll be exceptionally vigilant if the master cylinder does not fix the issue and I need to send the booster out for restoration.


Bill