Hi Guys,

This is my project Jag, it's a barn find, 2002 3ltr, with 33,000 on the clock.

I'm having issues around the brakes, the pedal is going straight to the floor. At some point in the past it's had new calipers and discs, some brake line work and it's clear the ABS and servo have seen a spanner...

I've replaced the master cylinder and ABS unit, but I'm getting no joy with brake pressure.  I've done a vacuum bleed, a manual bleed and a pressure bleed, flushed several ltrs of Dot 4 fluid through the system. I've isolated the rear calipers, checked all the pipework I can see, can't find any leaks or breaks.

What am I missing?  I'd be very happy to get it back on the road for Christmas, any ideas/suggestions?

 

Hi Tim, if there is no pressure resistance whatsoever, it has to be a leak somewhere in the system.

When you try to pump the brakes, does the system lose fluid and need to be topped up?

Have you taken the brake servo apart to check that is sound?

Have you looked underneath to determine if there are any patches of fluid on the ground beneath the car?

N6 JMX

Hi Tim

The brakes are a simple hydraulic circuit. Pressure applied at the pedal is transmited to the rear of the brake pads pushing them onto the disc.

So there are a limited number of reasons for your symptoms. Firstly the brake pedal is not compressing the fluid inside the master cylinder. If that part is working OK and there are no leaks in the system the hydraulic pressure must be passed to the slave cylinder. Unless that is seized the pressure will force the pads against the disc.

If all parts are free and connected with no leaks, applying pressure at one end will make the other end move.... unless there is air in the system somewhere. Air is compressible, brake fluid is not, so if you have any air in the system it will absorb the applied pressure rather than transmitting it.

No leaks suggests an air-lock somewhere.

Regards

John

thanks guys, no drips or leaks observed, can't speak to the condition of the servo, it has been sat for a few years so everything may just be perished. Other than this teething problem it's a fairly good, clean, low mileage machine. Should have plenty of life when it's back together.

Hi Tim

If the piston seal on the master cylinder or slave cylinder is perished it is possible that the fluid simply by passes the piston, so no compression. However you would expect that to be showing as a leak....

John

new front offside brake calliper and it seems to be happier, not an easy job nudging the machine back into rolling condition after a long stretch asleep. Thanks all for taking the time to reply, very, very much appreciated.

Hi Tim

Presume you have already had a look at the sills as these are hidden behind the plastic covers and are VERY prone to rust.
Mine had some rust but has had full repair plates welded to both sides and the whole underbody is now protected with Lanoguard.

Good Luck with yours

John

thanks John, yes mine appears rust free!

 

Still having brake issues though, it's throwing up DSC fault and locking calibers, although not always the same one. Rear passenger the other day, front driver today. Bit of a head scratcher. It drives with a soft pedal, then gets harder, until a brake locks on. Wait 30 mins and it'll move again.

Like triggers broom I'm replacing bits until it's an entirely new car. Might swap out the servo next.

Fortunately parts don't seem that expensive, and it's already more reliable than any of my land rovers.

But when it works it's great. 

Hi Tim

I love mine (27k and serviced every year!)

Your issue sounds like the pistons are sticking in the cylinders so not properly returning when pressure is released. It could well be if there was air and condensation in the system causing corrosion you may end up needing new calipers all around as well as a new master cylinder assembly to solve all of the issues. If it is half as good as mine though it will be worth it in the end 🙂

Good luck

John

On 12/23/2024 at 4:53 PM, Big John said:

Hi Tim

I love mine (27k and serviced every year!)

Your issue sounds like the pistons are sticking in the cylinders so not properly returning when pressure is released. It could well be if there was air and condensation in the system causing corrosion you may end up needing new calipers all around as well as a new master cylinder assembly to solve all of the issues. If it is half as good as mine though it will be worth it in the end 🙂

Good luck

John

thanks John, Yes I think you're on the right track, it had been standing for years, the last owner fitted some nasty chinese units on the back, so i'm feeling it's a mismatch of equipment and seized up kit. Master cylinder replaced. working my way around everything else. 

With the brakes locked up try loosening the master cylinder from the vacuum unit, if that releases the brakes you will need a new vacuum unit, 

It appears to be a common fault caused when bleeding the system and pushing the pedal fully to the floor (use a wooden block or similar to prevent excess pedal travel)