XJ40 3.6 spongy brakes

Hi, My brother has an 88 XJ40 Daimler we are bringing back to life.

It already had the SLS removed but not the brakes, so we did an XJ6 S3 brake booster conversions. We did not removed the master cylinder… simply moved it out of the way, then re attached it.

But we thought it good practice to belled all the brakes with fresh fluid.

All done and the brakes work, but they are spongy. We have put a lot of fluid through the system and it bleeds up easily and clean… but cant get a hard pedal,

Any suggestions please?


Sorry about my typos… doh!!!

power bleed the rears

Two things,
1 there is a pen icon lower right of the screen, this allows you to edit your post.
2 what brake fluid are you using, DoT4, 5 or 5.1?

Morro …

Just like Robin said, you can edit your own post using the pencil icon at the bottom of your post, but you have to be logged on for that to appear. So don’t be sorry about your typos … just fix 'em :cowboy_hat_face:

After you do the vacuum brake conversion the braking will feel a little different. It will take just a little more pressure on the pedal to get the same amount of braking as the old hydraulic system. Not that much but it is noticeable. Could this be your problem?

Thx Larry. Will try

Grooveman, its not the amount of pedal per se, its a spongy pedal no matter how hard its pushed. It feels like it wont get hard regardless… but it does stop the car

Morro …

Generally a “spongy” brake pedal means there’s air in the system which unlike hydraulic brake fluid is compressible and results in the pedal having that extra little travel.

But the real test is can you lock up the brakes so that the tires skid (or try to with the anti-skid system working) ? If you can’t do that in a panic stop you have a problem and need to investigate further.

I agree that carefully bleeding the brake system is the first step.

How does it feel with the engine not running and after several presses to bleed off the vacuum?

I suspect it is just the feel of the servo-assisted system, as opposed to the power brakes system that, I’m assuming, was installed.

If you have indeed replaced a power brake system, how have you reused the master cylinder, as it should only be feeding the front brakes.

I had a spongy feel to the brake pedal a couple of years ago when I changed both the front and rear pads at the same time. I knew it couldn’t be air in the system because I hadn’t disturbed any part of the hydraulics. It was quite worrying, it just didn’t feel right despite the car stopping perfectly but I used it like that for a couple of days and the spongy feel disappeared.

Ah. Thanks. Maybe persist a bit longer and see.

I’ll ask again, what BF are you using? If it’s DoT5 then there may be micro bubbles suspended.
If you have replaced the brake pads then they will need bedding in by doing a few hard stops 30mph down to <>5 mph

Robin, not sure what we used. We did it about 6 months ago and are just getting back to it. Got distracted by my Mark 2. Anyway, why would that make them spongy? We most likely used DOT4 since thats what I have in my shed. Cheers

Hi Grooveman,
I don’t think so… these are very soft. I may need to do some more symptom identification first… like pedal fade and the like. cheers

Hi Dieselman,
we retained the XJ40 master cylinder. It only drives the front brakes??? What drives the rear?? cheers

Thanks Gents,
so I have some more testing to do based on the comments made so far. I will do these and come back.

Thanks again

Hi Robin. I think its Dot 4 and we didn’t replace the pads…yet… Just getting it going so we can assess its viability as an on going project… If I cant sort what we have I may just do it all again with a vacuum bleeder, then we will know. I assume Dot 5 or later is not the right way to go for these old Jags? cheers

Hi Abercanadian,
will do. Why just the rears? Thanks

Sorry I meant DoT 5 this is silicon fluid and if you don’t let it sit it hold micro bubbles of air.
Just FYI you don’t need to reply to each poster in a seperate post, just make one post with all your answers/replies.
You might find you have better results doing a power bleed rather than a vacuum bleed.
This is the system I used, an old bike inner tube and a 12v compressor;

Now…nothing, it won’t be working.

With the power system the front brakes are pressurised by the master cylinder, the rears by the high pressure pump only.
You can’t mix and match parts from a power brake system and a servo-assisted system, they are completely different.

Why did you swap the power system?

Plough your way through the Teves III power brakes thread to see how it functions. The Girling system is basically the same operation.
Teves iii braking system failure - XJ-S - Jag-lovers Forums