Acceleration when stepping on brake plus hard braking, need car tomorrow!

[MariaAZ]

We just had a problem develop with our '74 260Z. When the brake pedal is depressed, the engine revs. The brake pedal is also very hard and it takes a lot of strength to push down. Any thoughts/solutions would be greatly appreciated, as this is currently our only running vehicle.

[jmortensen]

Brake booster is leaking, so when you step on the brakes you're getting a vacuum leak letting more air into the engine which speeds it up a couple hundred rpm. Because you have a leak, there isn't as much pressure differential on either side of the diaphragm, so you don't have as much power assist. If it's really bad you won't have any at all.

[Travel'n Man]

Odd's are you can order a rebuilt brake booster from NAPA or one of the other major supplies. I doubt they would have one of those on their shelves - they will have to order it from a warehouse if they have a rebuilt one at all...........best of luck. Personally I would refrain from driving with no power assist or the risk of loosing your power assist!

[cygnusx1]

Or...block off the vacuum hose and drive it without power assisted brakes temporarily, if you need to for emergency purposes. It can be done safely. Just be very vigilant and aware of the increased braking efforts.

[MariaAZ]

Thanks for the input. I borrowed a car for today and have a couple days to figure out what to do. I've got another Z I can pull the booster out of until I can get another one, and I've got a few manuals to help. From the experienced voices, how difficult would you say the remove/replace process is? I've done a bit of wrenching under the hood, but never anything with brakes beyond replacing the master cylinder on one of my Roadsters.

[FastWoman]

In addition to the brake booster issue, your engine is running too rich (which is why it runs faster when it gets extra air).

[MariaAZ]

I disconnected the vacuum line and plugged it at the manifold and the car runs fine. It brakes fine as well (takes more effort but Hubby, who drives it, says it's not really bad) so we've got a bit of time.

Could it be a faulty check valve? I read a reference in one manual that the check valve may cause hard braking problems.

[robert84]

I would refrain from driving with no power assist or the risk of loosing your power assist!

[beermanpete]

I disconnected the vacuum line and plugged it at the manifold and the car runs fine. It brakes fine as well (takes more effort but Hubby, who drives it, says it's not really bad) so we've got a bit of time.

Could it be a faulty check valve? I read a reference in one manual that the check valve may cause hard braking problems.

The check valve is not the problem. All it does is prevent loss vacuum assit when you floor it (low manifold vacuum) or if the engine stalls while driving which allows you a shot or two at the brakes with assist so you can stop the car safely.

You need to rebuild the Master-Vac (which has alreay been said).