What makes the brake pedal sink down??? Help me please!

[Black1987Stang]

As you can tell from the title when im on the brake the pedal slowly bleeds down to the bottom and ill have to let go , let the pedal come back up and press it and it repeats...Sometimes though it will be nice and hard like a normal brake. Is this the master cylinder or maybe check valve?

 

[85_SS_302_Coupe]

Master cylinder or maybe a leaky line.

 

[BlownFiveLiter]

Sounds like you have air in your brake system somewhere. You're not noticing any brake fluid leaking anywhere, right?

 

[Black1987Stang]

Nope no leaky fluid or anything. Its not like a spongy pedal like when theres air in the line, it just sinks down. For the first few seconds its fine then it starts bleeding down and ill have no brake power. I no older Fords are bad for master cylinders so im thinking its that.

 

[speedytang]

Check valve on the master cylinder usually will cause this by letting the vacuum to enter back into the engine.

 

[Black1987Stang]

Check valve on the master cylinder usually will cause this by letting the vacuum to enter back into the engine.

Im pretty sure you mean the check valve right, is there a way to test one to see if its bad or not?

 

[jrichker]

You have an internal leak in the master cylinder. Fluid is escaping past the rubber piston seals and returning to the reservoir.

 

[93lx408]

replace the master cylinder and be sure to bench bleed the new one b/f putting it on the car

 

[Euphoric306]

exactly, good excuse to get a 94+ master cyl for that future rear disc swap

 

[Black1987Stang]

OK so its the master cylinder....Thanks everyone, Ive done a master cylinder on a gmc truck once, all i got to do is fill it with fluid and keep pressing the plunger until fluid runs out of both brake output holes? Can this be done on the car but with the brake lines not connected since i dont have a vise to hold it in? I can get one with or without a reservoir, which one should i get? the one with the reservoir already on it right, so i dont have to swap out my reservoir?. Do i use DOT 5 fluid i forget. I think it says so on the cap though. Do i have to bleed all the brakes also? Thanks alot

 

[jrichker]

OK so its the master cylinder....Thanks everyone, Ive done a master cylinder on a gmc truck once, all i got to do is fill it with fluid and keep pressing the plunger until fluid runs out of both brake output holes? Can this be done on the car but with the brake lines not connected since i dont have a vise to hold it in? I can get one with or without a reservoir, which one should i get? the one with the reservoir already on it right, so i dont have to swap out my reservoir?. Do i use DOT 5 fluid i forget. I think it says so on the cap though. Do i have to bleed all the brakes also? Thanks alot

Use DOT 4 fluid to be compatible with what you already have.

You can bench bleed the master cylinder on the car without the brake lines connected.

You will have to bleed the system, front & rear when you are finished with bench bleeding. Anytime you disconnect a brake line, you let air in the system.

Master cylinders with the reservoir are generally more expensive than without the reservoir. It is a pull, tug and push operation to swap it from one master cylinder to another.

 

[FORCED2DV8]

Use DOT 4 fluid to be compatible with what you already have.

You can bench bleed the master cylinder on the car without the brake lines connected.

You will have to bleed the system, front & rear when you are finished with bench bleeding. Anytime you disconnect a brake line, you let air in the system.

Master cylinders with the reservoir are generally more expensive than without the reservoir. It is a pull, tug and push operation to swap it from one master cylinder to another.

I AGREE

 

[Euphoric306]

go ahead and use DOT 5... let us know how that turns out






jk there, DOT 3 and 4 are compatible, but do not mix at all with DOT 5, it will ruin your brake system. DOT 5 is higher temperature resistant, but collects dust and stuff a lot quicker than 3 or 4, and needs to be changed every few months i believe, if not its a helluv a lot more often than 3 or 4. DOT 5 is mainly for race only cars.

good luck, easy swap but if not done right can have nasty effects

 

[Black1987Stang]

Master cylinders with the reservoir are generally more expensive than without the reservoir. It is a pull, tug and push operation to swap it from one master cylinder to another.

So i can just pull off the reservoir and push it down on the new master cylinder until it snaps in?

 

[Shakerhood]

Yes the reservoir can be removed from the old one if needed.