I have a 93 ford ranger. The brake pedal started going to the floor slowly then after a few days it goes to the floor as soo as you step on it with engine running. I have checked all wheel cyls no leaks i have checked master cyl it is full. Replaced with new master cyl same results. v/6 engine with power brakes Could it be vacuum chamber? Thanks

 

Before you change the vacuum assist, check the gromet where the vacuum line enters it. It could be dried out and cracked. This can leak and cause the problem you describe.

 

Yep, had that happen after panic stopping my 1st Ranger (2.3L) years ago. I was stumped until I noticed the fitting popped out. I understand worn motor mounts are probably the chief cause, so excessive engine rocking could also accelerate the grommet wear & tear.

 

Thanks for the info will have to wait till mon till i can try it will post with results.

 

When I had a power booster go out, it made the brake pedal really hard to push, it wouldn't go to the floor. It was hard like pushing down on a brick.

 

If the pedal is going to the floor, it has nothing to do with the vacuum booster. It will have a full pedal if it has fluid, and will be hard to push as Wendell said. It has low fluid and probably air in the lines. If you can pump it up, it has air, and the brakes need bleeding.jd

 

If the booster line is off or leaking, it'll seem like the hydraulics are out. Believe me, Jim. Been there, done that. Nearly wrecked some poor guy's little car twice when I wasn't prepared for it in that '88 XLT way back when.

 

Happened to me in a 91 Explorer. Definitely one of those "What the @#%$&^**" moments.

 

I'm gonna have to check mine out-don't wanna have one of those "What the@#%$&^**" moments....

 

To be clear:

Vacuum Booster: Uses engine vacuum against a diaphragm to ASSIST the driver in pushing the piston in the master cylinder. If there is a leak or the vacuum line becomes disconnected you will instantly be thrust back to the early 1900's, but you will STILL have FULL braking power at the wheels assuming you are strong enough to still move the pedal. The power assist system is completely isolated from the hydraulic system.

You either have:

1) Broken steel line creating a leak. (unlikely as you stated the fluid level is not changing)

2) Air or water in the hydraulic system.

3) The rubber hoses going to each front wheel and the rear axle are going bad and "swelling" on you.

You did say that you replaced the master cylinder, did you bench bleed it before installing it. If you did not bench bleed it you may have air in the master cylinder itself.

How did you go about bleeding the rest of the system after you replaced the MC? Old method of pumping pedal or did you use a vacuum pump at the wheels?

 

Hey BigBlock, try driving it with the check valve removed from the booster grommet. Dare ya.

 

So, let me try to get your logic straight. If what you are saying has any truth at all then why doesn't my pedal go all the way to the floor if I press it with the engine off? Engine off produces no vacuum which would produce the same results as a leak or disconnected booster hose. Funny thing is that when I push the pedal with the engine off there is about 20 times more resistance than there is with the engine running. As far as the check valve you speak of - it is a safety device that allows the vacuum booster to retain some vacuum in the event that the engine stalls, allowing the driver to have power assist brakes for a short period even without a vacuum supply.

I would have no problem driving around all day long with no vacuum line attached of no check valve in the hose. Without the check valve I will notice no difference as long as the engine is still creating vacuum. Without the hose attached to the booster I will lose all benefit of the vacuum assist, but will still be able to stop the car in the same distance providing that I have the leg power to push the pedal hard enough. The worst thing I will endure is a very sore leg the next day, but altimately I will be fine.

Read these if you still don't believe:

HowStuffWorks "How Power Brakes Work"

Troubleshooting Power Brakes

 

I don't know if they've altered the design, but it doesn't work that way in practice. Don't really care why. If the hose comes off, your pedal goes almost to the floor before your get anything useful to press on. Maybe plugging that hole with a bolt would restore the mode you expect. I'd cut open a booster if we had an extra lying around.
I will NEVER forget nearly rear-ending a little car twice in a row with my 1st Ranger. Drove it to work, looked everything over at lunch & the only thing I found was that hose pulled. Then I was about kicking myself for not seeing it right off. I had driven the truck into the factory between the saws for the big sodium lights. Hooked it back in & worked perfect for years until my brother backed into the truck.

 

Bigblock is correct, if the vacuum line becomes disconnected the pedal will get hard and the brakes will get suck. The pedal WILL NOT drop. If it does, you have problems unrelated to the vacuum booster. If you disagree, I highly suggest that you either do a search regarding the function of a brake vacuum booster. Or, better yet, just take yours off and take it apart. You will see you are wrong. I have rebuilt hundreds of brake boosters so I understand the function and mechanics very well.

 

In every booster since the beginning of time has a direct mechanical link with the master cylinder it self right from the brake pedal. I've disconnected the whole assembly off my 87, and the pedal feels just like if the engine were off.

More then likely you were panicking that you didn't have much braking power so it SEEMED that your pedal hit the floor (like a total brake failure) because your truck wasn't slowing down as quickly as it normally would, or you had a brake problem elsewhere, and a working booster was just hiding it.

Automotive brake systems are meant to be as failure proof as possible.
They would not put the time and money developing the dual piston, dual reservoir master cylinders, low brake fluid lights, etc.. if a simple failure or removal of a check valve in the booster would completely disable almost all braking.