Brake Booster Issue

Hello all, first time poster here.

I did a search and didn’t find a previous thread that sounded like the issue I’m encountering, so here goes.

1969 XR-7, 351w 2bbl, FMX, midland brake booster. Original booster on the car was un-serviceable. Ordered a new midland booster from NPD. Installed the booster, brakes work excellent. However, there is a constant audible hissing sound from underneath the dash when applying light to moderate brake pedal pressure, IE normal braking. If you put some effort into the pedal, the hissing will stop. There is no noticeable impact on idle speed or quality when applying the brakes.

Could this new booster be faulty? What’s really throwing me for a loop is that the brakes work very well, so the booster is doing it’s job in a relatively satisfactory manner, however the hissing sound is at the very least annoying. Any pointers on what to look for? My thoughts are some seal at the firewall isn’t quite doing it’s job. Any advice would be appreciated. Thanks in advance.

I had a similar issue that ended up being a faulty booster. Started with the hissing then got worse, it was an obvious seal leak.

Do you still hear the hissing when the car is stopped? Does the hissing stop once the pedal is depressed? Does the pedal travel slowly to the floor once you start to depress?

The hissing will stop if you put a little extra pressure on the brake pedal than necessary to keep the car from rolling in drive.

Pedal travel feels pretty normal to me honestly. I’ve got a ‘69 mustang also with a midland booster and I’d say the feel of the pedal is similar.

Only other thing that came to mind is if you potentially knocked another vacuum line out while getting under the dash to install the booster. But since the hiss stops when the pedal is depressed, I think we can rule that out.

I know what a pain it is to physically replace the booster, not to mention having to bleed the brakes all over again after. However, if it were me, I’d call up NPD and have them replace it. If you can already hear the hiss on a new/recently rebuild unit, chances are it will continue to get worse. No reason you need to deal with that.

Mine eventually got to the point where I had to slip my foot under the brake pedal to pull it back up. YIKES!

1969 disc brake pedals and brake booster push rods are specific to 1969.

1970 disc brake pedals and brake booster push rods are specific to 1970.

They cannot be mixed, or you could have a vacuum leak like you are experiencing. Mixing parts will push the rod at an angle and cause failure of the diaphragm in the short order. Is there a chance that one of those components could be incorrect for your application?

There’s the difference that CougarCJ referenced.

This is just what happened with my ’70. The restoration shop that upgraded my manual drums to power discs used a 69 booster and pedal. Aside from the hissing, I wouldn’t get any braking power for the first half of pedal travel, then when the angled pushrod finally engaged, it would just about lock up.

Swapping to a non factory installed booster (was Bendix, now Midland) there is a chance the spacers are an issue. Bendix has spacer attached to booster body via one stud w/ a flange lock nut. The Midland has a plastic & cast alum spacer/ breather. Combining or deleting them can cause this issue.