[jerryt]

I have a 1974 nova. A few nights ago the brake pedal started sinking to the floor. But when I pumped up the brakes three times the pedal returned to normal. I thought it was a vacuum leak so I replaced the booster fitting and inspected the vacuum hose and grommet. All where rubber parts where fine. I am going to pull the wheels off and inspect the brakes for leaks. If no leaks I will bleed the brakes. Last resort is the master cylinder. I am leaving towards the master cylinder. If no leaks or air in the lines then it's the master cylinder? Please any information will be helpful.
Thanks
Jerry

[Big Dave]

Brakes going to the floor is due to air in the system/ Air is compressible, hydraulic fluid is not. I would have your brakes power bled at Brak-o or a similar nation wide brake repair shop. If you think you have a bad master cylinder perhaps you woulf want to buy this tool that cost as much as a professional power bleed, but you can do it yourself as many times as you want in the future for one purchase price.

https://www.amazon.com/Motive-Produc...40413480&psc=1

Air is entering your system. It could come from a leaking line (rust through), a bad bleeder on a wheel cylinder, a bad wheel cylinder seal, or a bad master cylinder seal.

Big Dave

[jerryt]

Thanks for info Big Dave. Will take wheels off and look for leaks. But the part that baffles me is it brakes fine and then about 5 minutes into test drive the pedal bottoms out at complete stop. Brake system has been dead perfect for years and then boom. Will let you know what I find out

[Big Dave]

Just a wild guess I would say the main seal in the master cylinder died. Especially if you are using DOT 3 or DOT 4 fluid that adsorbs water and is highly basic (high Ph) which causes corrosion. You are supposed to drain and replace your brake fluid every five to seven years to rid your system of water and rust particulates.

Big Dave

[jerryt]

Will still check for leaks and bleed brakes. If not will replace master cylinder. Any tips on draining brake fluid?
Thanks
Jerry

[jerryt]

Well I took the tires off of my nova. The rear where cylinders are dry. Rear brake lines are also dry. Checked front caliper they are dry and the hose is dry. I bled the brakes no air. I test drove car the brakes work. But at a stand still with the foot on the brake the pedal starts to sink. I read if you pump brakes three times and with pressure on the pedal start the car the pedal should go down and the booster should be ok. So that leads me to the maste cylinder?

What do u think
Jerry

[Elvis]

jerryt,

There's a special tool for working on brakes called a "Line Lock".
It looks like a C-clamp, but its specially designed to safely clamp brake lines without breaking them.
You need to line lock the two hoses for the front brakes and the one hose for the rear brakes, then step on and hold pressure on the pedal.
Still goes to the floor?
Bad Master Cylinder, guaranteed.
Quick and easy check that's 100% correct.
Check with your local auto parts store to get some, or check with your local tool trucks. One of those guys will be able to help you.


Elvis

[69Nova350]

If you did not remove the brake drums after you pulled the wheels, you may have missed a leaking brake cylinder. Initial leaks may result in fluid inside the rubber boots on the ends of the brake cylinder. After removing the crude over the boots,(don't breath it) look for fluid inside the boot by gently peeling the boot back to look inside with a flash light.


I think that if the booster were failing, you would have to pump them up to get pressure. In my experience suspect the master cylinder internal rubbers have worn out over time. You have several choices: 1) remove and replace the master cylinder (with new or re manufactured) remembering to bench bleed it before installation or 2) rebuild the master cylinder including inspecting and honing the cylinder if there is no rust or significant scaring.



Consider some penetrating fluid on the hydrophilic brake line fittings. Use a flare nut wrench if you have one to protect the hydrophilic brake line fittings which are often mangled during removal.

[Elvis]

Originally Posted by jerryt

Well I took the tires off of my nova. The rear where cylinders are dry. Rear brake lines are also dry. Checked front caliper they are dry and the hose is dry. I bled the brakes no air. I test drove car the brakes work. But at a stand still with the foot on the brake the pedal starts to sink. I read if you pump brakes three times and with pressure on the pedal start the car the pedal should go down and the booster should be ok. So that leads me to the maste cylinder?

What do u think
Jerry

JMHO, but yes, it sounds like you have a bad master.
Do the line lock test I described in my other post, to verify.
Also, 69Nova350 had some very good advice with pulling off the boot and checking the wheel cylinders on the rear drums.
A very small flat tip screwdriver (like the kind that you can clip it to your shirt pocket) or a pick will work. Just pull the end that attaches to the wheel cylinder down and see if any fluid runs out or if you see moisture inside.
As you said, should be dry, but if you didn't pull the boot loose, it may be leaking and not show.
Please use care when pulling that boot loose, so you don't tear or rip it.


Elvis

[Big Dave]

From fifty years of experience I would say if your master cylinder is bad the wheel cylinders will be next to go. this is due to the fact that brake fluid absorbs water. Water and aluminum pistons in a cast iron bore is a recipe for failure.

Rebuilt wheel cylinders are cheap. You need to flush the lines to get rid of the old fluid. So why not bite the bullet and do a full break job at the same time (don't have to change the shoes unless they have brake fluid on them or they are worn out).

Big Dave

[Elvis]

…+1 on Big Dave's comments...

[62 NovaWagon]

Originally Posted by Elvis

…+1 on Big Dave's comments...

+2 on that..

[Elvis]

Jerry,

Just to be clear, if you find the wheel cylinders do need replacing, that doesn't necessarily mean the master is ok.
You still need to test it, regardless of what you find down the line.
Could be a case of multiple problems.