XJ6C booster replaced, still have hard pedal
 

Hello again,

I replaced the booster on my 76 XJ6C because I had a hard pedal. Before replacement there was no difference in the pedal, engine running or not. Now, with the new booster, if I hold the break down and start the engine, the pedal moves toward the floor about anothed 1/2 an inch which I believe is normal. However, brake pedal is hard when braking. I put a "T" in the line between the one way valve and the booster with a vacuum gauge attached. I get around 20" Hg when the engine is idling. As soon as I apply the brake (with engine running) vacuum drops to 5'' Hg and holds steady. Releasing the pedal brings it back to 20" and steady. Turn the car off and the gauge holds at 20". I went away for about 15 or 20 minutes and it was still 20". I then pressed the pedal (engine is still off) and lost all vacuum. This is a brand new booster (not rebuilt). Did I get a bad one? Removing and replacing was a real pain, the pedal housing needed removal, hoping I don't have to do that again...

This was my first booster replacement on an XJ6. Any rookie mistakes I might have made to cause a hard pedal? Any help will be greatly appreciated.

Steve,

76 XJ6C

Hello, Steve,
You wouldn't be the first person to get a faulty 'new' part. Permanent vacuum drop is pretty much a dead give away that the booster is faulty.

It's time to try out that 90 day warrantee the 'new' booster should have come with.
(';')

This is stretching the memory real far.

Up in the RH fender front section, basically under the headlamp should be a vac tank. Your uo there, so about teh size of a gallon can.

This is the reserve vac, and the design was to give about 4 brake pedal applications in the event of an engine stall whilst in motion, so the car could be safely stopped. In practice, NO idea.

It has NOTHING to do with the actual BOOST, just a reserve of vacuam.

One of my S2 cars had similar, and the brake fluid was so old it was no longer brake fluid. A quick flush and the stopping oomph returned.

Some have reported a jammed caliper, which I reckon would be the rear inboard in this case, and that will give that scenario.

In all the Jags I have had, I have never replaced a booster.

Grant,

My cannister is not hooked up. I thought that cannister was for removing fumes from the fuel system when the car shuts off? New front calipers, rotors, pads and brake hoses. New rear pads. I flushed out the brake lines with a vacuum and nothing seems to be hung or stuck. The car stops straight and true from high speed, I just have to use all my leg strength to stop.

Thanks,

Steve

Nah.

The fume canister is on the LH side.

The vac tank on S2 is a big sucker, and kind of angled in there on the RH side in front of the wheel, and bolted down. NOW, thats the Aussie cars, you guys got some very different stuff, but I reckon that item would/should be there.

Was the new booster actually for a S2 Jaguar???. I ask this coz down here I have heard/seen, the likes of "thats the same as ......." or ........." when in fact they LOOK the same, but the innards are very different, and that is in the boost that is able to be applied.

On some models/spec/markets, there is a "pressure differential switch" down on the RH inner guard, just near the chassis rail, it has 3 brake pipes plus an electrical switch. If that has tripped, then wierd pressure stuff happens. If its there, disconnect the wires, and unscrew the switch VERY SLOWLY, and in a QUIET place, and LISTEN CAREFULLY for the "click". Once that "click" is heard retighten that switch, and reconnect the wires. Dont rotate it too far so it falls out, that then gets messy. I have done this many times, and NEVER had one come out, so it is doable, just take your time.

Or:


Check the system as to it's ability to hold vacuum. Remove the source pipe at the engine. Plumb in a Mitty Vac. neat little tool to add to your stuff. Manually pump it up. 20hg's or so. See if it leaks away. A set of AC gauges and pump might be rigged to do this. I've not tried either, so.


My car is a molested S3. But, the brakes are all Jaguar. No vacuum reserve tank for the brakes. There is one for the AC. It is under the booster of my LHD car.


The canister for the vapour recovery system is in front of the RF wheel. All Jaguar again.


But, each system operating on the vacumn developed by the Cadillac LT1 engine.


Just check the lines and think about the logic of the design.


Carl

Steve,
I notice you're in New Jersey, therefore have a US Emissions car like mine which is also 1976 MY.

Our vacuum "storage" can is hidden in the left fender outboard of the booster. You might have to look closely to see it. I've understood this was only for AC flapper operation after the car was shut off.
https://cimg5.ibsrv.net/gimg/www.jag...800d1ba549.jpg
The hose nipple on my can is very rusted and doesn't seal very well


US cars of that era were required to have a Vapor Recovery can, fulled with charcoal. Its location is under the Right fender just behind the headlights.
https://cimg1.ibsrv.net/gimg/www.jag...3aa9bd4332.jpg
Topside this has one port open while the other is connected to the top of the carburetor.

I've found no other object that could be a vacuum storage device.
(';')

I hooked up my Mytyvac, as Carl suggested, pumped it up to around 16" or 17" Hg and it holds vacuum just fine. BTW, It takes a lot of pumping to pull vacuum from the booster, but eventually it does come up. After 10 minutes or so I checked to see if I still had vacuum, I did. I pushed the brake pedal and vacuum went to 0 immediately. From what I have read recently, it should not do this. It should use up a portion of the vacuum on successive pumps of the brake. Say 2 or 3 pumps, not just 1. This kinda goes with what Grant was saying, if the additional tank were plumbed, then I should be able to pump the brakes 4 or 5 times before losing all vacuum.

One other thing I noticed, when the engine is up to temperature, applying the brake causes the engine RPM to move from around 700 RPM to 900+ RPM. If the carbs were running a little rich, then maybe vacuum leaking through the booster is leaning out the engine and causing RPM to rise?

I'm really starting to think the booster has a torn diaphragm.

Thanks,

Steve

While waiting for another booster to arrive, I realized I could pull the booster from my 73 parts car and replace my questionable booster in the 76 with it. The bolt pattern on the older booster is not the same but I realized I could swap the pedal box and booster as a unit and at least test it. And voila! brakes work perfectly. Another benefit was that the engine idled down to 500 RPM. So I got out the uni-syn and re-tuned the carbs and now get a very smoooth idle at 800 RPM. So the booster in question was not only denying me good breaks, it was leaking vacuum (ever so slightly) and interfering with the carb tune.

Thanks to all!

Steve


76 XJ6C

Well Alright then, Steve!!
So you got a faulty 'new' part.
Did it come with that 90 day warrantee I mentioned earlier? Did you use it? Did they honor it?
(';')

I've found that having the brake booster rebuilt is the assured way to get back on the road....