pedal goes to the floor on the first pump?

[IncorpoRatedX]

i've got a 76 sedan and a 73 wagon, both have disc front and drum rear, both do the exact same thing.

first time i push the brakes, if i push slowly, they will go all the way to the floor, second time they'll have pressure, i've bled both cars systems, dont have any leaks, can't seem to sort it out.

I was first suspecting the vaccum check valve on the line coming from the manifold to the brake booster, but both 'seem' to be working.

Anyone else have this issue? how'd you resolve it?

[Scorpma82]

are they std brakes??

Did you use a silicon based fluid or a Glycol based fluid (Glycol is DOT 3,4 and 5.1........Silicon is DOT 5)

[IncorpoRatedX]

STD brakes? standard brakes? they're stock stuff, so yeah, i guess?


I used DOT 3

[davetrees]

I used to have a similar problem (although not quite as bad as pedal going to foor ... just soft on first application).

If it's going to the floor, you have a MAJOR amount of air in the system somewhere ! Might even be sucking air, if not leaking fluid ?

Things to check (not in any particular order) :
- brake hoses ... especially the rear one from body to axle housing. They go "internally soft" even if not leaking, and expansion causes loss of brake pressure. They can also get fine cracks that will suck air but not leak fluid. Putting a new hose here resolved 80%+ of my brake issues. Also make sure the front hoses are good.

- Fluid leakng into booster ? (if you have one)

- Rear shoes not adjusted up far enough ? (the auto adjusters are crap .... have the shoes as close to the drums as possible without dragging).

- wrong fluid is a possibility. These cars (and their seals) are designed for DOT3 fluid. Some DOT4/5 fluids can make the various rubber brake seals go soft & leak.

- Have you rebuilt your calipers (new seals) ? If not, do so. Same with rear brake cylinders.

- Front pad knockoff ? Check wheel bearings.

Hope that helps

[IncorpoRatedX]

all four corners have new brakes, all adjusted good, bearings are right and tight. All the hoses are old, possibly original, i will look into those!

But i guess i should say the pedal doesnt just drop to the floor, if i push slowly i can push to the floor, brakes still work, just not as well, give it a second pump or even a sharp quick pump the first time and it's firm.

[Scorpma82]

if the hoses are old.....the rubber is flexing.....replace them all rubber hoses (much like radiator hoses expanding under psi)

[jvflash]

Brake master cylinders have dish shaped seals in them that actuate the primary and secondary hydraulics. A combination of fluid pressure and piston force, force the edge of the seal against the cylinder wall. The dish shaped seals wear on the edges and when that happens slow pressure on the pedal allows the brake fluid to bypass the edge of the seal and flow behind. The fluid however does not necessarily leak pass the rear seal as that seal could be intact.

When the brake pedal is released the brake fluid can readily pass back past the seal because the dish shape allows the seal to flex inward on return, stopping the possibility of hydraulic lock on the return stroke.

A sudden application of the pedal has the effect of pushing the piston inside the seal hard forcing the dish to expand rapidly making a better contact with the cylinder wall.

Two reason why this occurs -
1) Brake fluid is hydroscopic, that is it absorbs water, normally through the permeable rubber hoses, or loose brake fluid caps. This causes corrosion in the master cylinder which allows fluid past. However this would give a constant spongy pedal feel.
2) worn seals which still expand quickly under quick application.




Good news is that seals are cheap and brake hones are cheap too and fit in a drill. So easy fixed.

[IncorpoRatedX]

Even better news is that I have two brand new master cylinders sitting on my shelf!

Thanks for that info!