I have a question about a brake issue. Series III car. I get in and put my foot on the brake pedal and it feels firm. I start the car and the pedal immediately sinks about halfway down and then it stops and feels fine (doesn't continue to sink or feel soft). Once I'm underway everything is fine, no sinking, spongyness, and it stops great. The brake fluid resouvor is not showing any consumption of fluid and no leaks under the master cylinder or at the wheel cylinders. I've worked on brakes all my life but never experienced this before. I'm thinking maybe the vacuum booster is going bad. Parts are readily available and I have no problem working on it, i just don't want to go replacing parts until I get your input. Thanks.

Jeff

 

Have you removed the vacuum hose to the brake booster to see if there is brake fluid being sucked into the inlet manifold?

 

Just went out to check. Right where it comes out of the booster it is dry as a bone.

 

Normal with a vacuum booster

Try starting your car without your foot on the brake, wait a moment or two, then press the brake. It should feel normal

Cheers
DD

 

It may be the vacuum booster going bad, but that typically results in a very 'hard' pedal.

 

Sounds just fine to me.

No need to worry.

Carl

 

Just went out to try your test, Doug. Started and ran the engine for a couple minutes and then tested the brakes. Pedal felt normal, no sinking. Just to clarify, it started doing this recently and my 87 series III works just like a normal car. So something changed. Back in the old days before power brakes and only one braking circuit, if a wheel cylinder or line broke, the pedal went to the floor and you sailed right through the red light or stop sign. A mega-dose of adrenaline surges through your body and hopefully no one gets hurt. I hope to never have that feeling again. So I think I'll order a new booster. The old saying, better safe than sorry.

 

In addition, I think the master cylinder is as likely candidate as the servo/booster. If the seals are failing a touch, under the boost assistance the seals in the master cylinder do not quite seal until they have traveled a bit down the bores. Then once settled they work OK. Maybe wise to do both ?

 

TIy beat me to it, Doug !

 

IAs to brakes or the lack thereof, I go way back. Circa 1950, I got my first car with decent brakes. A 41 Studebaker Champion tudor.. Hydraulic brakes, my prior cars were all mechanicaly braked.
Many techniques needed to stay out of trouble. Including always looking for an escape route!1
Even wit it's simgle cylinder master, it was far better...

Carl. . .

 

Starting the process to replace the booster and master cylinder. The book says to remove the "pedal box" first. That is the aluminum housing where the brake pedal connects to the back of the booster. You have to stand on your head under the dash to get the pedal box out. I would like to not have to remove the pedal box. I got the master cylinder out and removed the 4 nuts on the back of the booster so it is loose now. The problem is that the booster is hitting the inner fender and there isn't enough room to get it out. I removed the hood latch mechanism as well. I have wiggled and tugged every which way but it won't come out. Before I get under there to do it like the book says, has anyone got the booster out without removing the pedal box? Thanks for your help.

Jeff

 

Yes. I have.

At the end of the day I wished I had just done it according to the book

Let's hear what others have to say!

Cheers
DD

 

I love standing on my head under a car dash as much as the next person, but it wasn't that bad to "suffer" for 10-15 minutes. Finding the 6 bolts (1 is bigger than the other 5) and then being able to just lift the whole unit out with just a little twist was well worth the neck/back pain.

Dave

 

Same in the XJS! Why did jaguar not make the indent a inch longer?

 

Thanks for your encouragement. I got it out. In my case maybe 15 minutes of actual wrenching but 90 minutes of getting up and down and going around to get the tools needed. A mirror and a light on the floor of the car make it a little easier. My car is a 1979.5 so it has the fresh air vent knob down there so I had to take that out first. On the plus side, I can now clean back there under the booster where there was 40 years of debris. It should go back together much quicker. Thanks for your help.

Jeff