Is It My Brake Booster or Master Cylinder? Can't Figure This Out! 1966 Thunderbird Help!

[wick001]

I recently bought a 1966 Thunderbird with the 390 and it started having brake issues that I can't figure out. I've done tons of research on this and I still can't figure it out, maybe you guys can help.

The cars brakes never felt right, the pedal would go pretty much to the floor when braking but at least the car would stop. I couldn't lock the tires up even if I tried.

So one day, I started the car and the throttle got stuck open a small bit at idle maybe around 800-1200rpms, I went to put it in drive and had no brakes. I tested this a few times and I don't understand it. When the idle is high I have no brake pressure, goes straight to the floor. When I return it to normal idle, I have hardly any brakes at all but at least I can slow to a stop. What is this about?

1st thing I did was bleed all the brakes with a friend. Front disc pads are new, rear brake drums are adjusted tight and no leaks anywhere. This did not help at all. Same thing, no brakes.

2nd thing I did was replace the master cylinder with a Centric. Link: https://www.carid.com/centric/premium-b ... l=17058204

I bench bled it, adjusted the little brake booster plunger nut to actually push in the master cylinder a tiny bit when I installed it. Then I bled the driver front caliper until no air was left.

Now here is the weird thing. With the car off when I was bleeding the front passenger brake, I noticed I could get a full stroke of brake pressure, I could feel and see the pedal pushing a full stroke of fluid through the system. but it would take forever for the pedal to return to the top again. like 20 seconds.

Starting the car, same thing, pedal goes all the way to the floor again! I don't understand? I thought a hard pedal, not moving is a problem with the brake booster? With the car off, the pedal gets hard after a few pumps but again takes forever to get back to the top. With the car on, pedal just goes straight to the floor.

So I already replaced the master cylinder, the only thing it could be now is the brake booster or more air in the lines somewhere, or another bad master cylinder.

Why does it take so long for the pedal to come back with the car off? Why do I still have hardly any brake pressure when I adjusted the plunger screw so far out?

Please help!

[Terry64HT]

It sounds like your booster. I suspect that brake fluid has leaked into it and the diaphragm is now not assisting the pedal travel. The diaphragm helps to keep everything inside the booster in alignment, and if it is damaged or been eaten away by brake fluid then things don't move properly.
It also sounds like you have adjusted the pushrod nut at the booster too far. There should be a little bit of travel before it contacts the master cylinder, generally the brake pedal should move about 1/8" before the pushrod contacts the master cylinder piston.
Hope this helps
Terry
64HT

[RedBird64]

It's really easy to overstroke a new MC when bleeding it. You should not go to the floor during the process or you risk damaging the rubber plunger in the MC (it can travel past the machined bore and get hurt). It sounds like your new MC is bad now because nothing the booster does can cause the pedal to sink to the floor.

As Terry suggested, your old MC has probably leaked and ruined your booster so it will need to be rebuilt as well if it wont hold vacuum.

BTW, when your idle is high you have the most power assist from the booster.

Scott

[wick001]

It's really easy to overstroke a new MC when bleeding it. You should not go to the floor during the process or you risk damaging the rubber plunger in the MC (it can travel past the machined bore and get hurt). It sounds like your new MC is bad now because nothing the booster does can cause the pedal to sink to the floor.

As Terry suggested, your old MC has probably leaked and ruined your booster so it will need to be rebuilt as well if it wont hold vacuum.

BTW, when your idle is high you have the most power assist from the booster.

Scott

How do I know if it's the booster or the master cylinder? And why would a high idle affect the brakes at all?

What I don't understand is why pushing the brake pedal down regardless of the booster functioning properly or not is not pushing the master cylinder down enough to engage the brakes? Because it feels like the pedal is on the floor?

From what I've researched a bad booster is a hard pedal? I've checked my vacuum line and there is plenty of suction. I don't hear any leaks from the booster, and I took off the check valve and blew in the other end to see if it was leaking, it wasn't.

So it seems that the booster is functioning? but also, why won't my pedal return to full position/stroke? Could that be something in the booster not allowing the MC piston to return ?

[paulr]

Check to see that the transmission modulator is not leaking vacuum since it’s inline with the booster. Not saying it’s causing all your problems; it should be ruled out.

[jtschug]

How about 'all of the above'?

The engine is connected to the brakes through the booster. There is no other connection, so if something going on with the engine causes something to happen with the brakes, then it is the booster.

However, the booster only assists actuation of the master cylinder. If moving the MC doesn't cause fluid to move in a properly bled system with no leaks, then the MC is bad. It is possible for a part to be bad right out of the box, so the fact that it is new doesn't mean you should eliminate it from consideration.

[RedBird64]

JTSchug pretty much covered it.

To answer your other question, if you rev your eng to 8000~1200 RPM, there is more vacuum available for the booster to work with.
Also, Don't blow through anything associated with the booster. It works off vacuum, not pressure. You need to apply vacuum to the booster and watch for its ability to hold that vacuum. Your not going to hear anything. Before I had a hand vac. pump, I'd just use my mouth to draw a vacuum and then seal the end of the hose for a few seconds. When you remove the seal, it should release the vacuum.
One thing is for certain, unless you have a leaking wheel cyl or caliper, your MC is bad.
Whether it was born bad or was destroyed by over travel during the bleeding operation we'll never know but it's toast now.
Next time you bleed it, place a piece of 1X4 under the pedal so it doesn't travel too far. When you bleed the MC on the bench, dont push it all the way. Only push it about 3/4 of its travel (3/4" tops). If you push it so far that you feel something like a detent, you're trashing the piston inside.

Good luck!
Scott

[wick001]

I decided to have the Brake Booster and original Master Cylinder rebuilt through Ecklers. The brake boosters for the 1966 are impossible to find and apparently must be sent out to be rebuilt.

Here is the link: https://www.macsautoparts.com/ford_thun ... 65-66.html

I will keep you updated during this process.

[RedBird64]

Yes! All of the 60's vintage Tbird boosters are hard to come by. I could have exchanged mine at Booster Dewey's but when I talked to him he said if mine wasn't rusted or pitted, I'd be happier just getting mine rebuilt.
Apparently when you can find them, they're rough.

S

[wick001]

Well guys,

I finally got my brake booster and master cylinder rebuild back, and guess what. I now have brakes!

I still don't know what was causing all these issues, but it obviously was something inside the booster. Possibly the spring inside not returning the pedal. Who knows.

Most importantly, I now have brakes!

[RedBird64]

Sure nice to get something like that all ironed out eh?
Congrats!

S

[Mike Tbird]

I have a 66. Had a fantom brake fluid leak. No signs of fluid anywhere. Brakes were slow to release for while. Then no brakes and pedal went to the floor. Changed the MC but they were still slow to release. Got booster rebuilt by Booster Steve. Now the brakes are a little agressive but all the issues seem to be gone. I know they don't fail often but check the one way valve on the booster housing. Mine was also bad. I would have liked the brakes to be a little less grabby but all is working well.