P38A Why brakes don't seem right

Montyjohn · Sep 25, 2022

Hi, my 2000 4.0 has weird brakes.

Going from completely empty to fully pressurised take about 20s which sounds good to me. So I'm guessing the motor is good.
But, every time I press the brakes the motor runs for about 3 seconds. Is this normal?

Then, with the ignition off I get almost no brakes on the first pump. Pedal goes all the way down.

After ignition on, 20s later brakes work fine again.

I assume this isn't normal. Which bit do I need to replace?

kermit_rr · Sep 25, 2022

Sounds like the accumulator has expired. That's the round black ball near the battery. Easy to swap, just over £100 for a new one.
Pump should run every 4 presses

Montyjohn · Sep 25, 2022

kermit_rr said:
Sounds like the accumulator has expired. That's the round black ball near the battery. Easy to swap, just over £100 for a new one.
Pump should run every 4 presses

Ok thanks. It looks like it might be the original based on condition.
Looks like the previous owner was trying to solve something as the pressure monitor switch looks brand new.

I've noticed that they seem to be £180 new so inflation has clearly hit. Or £85 on an exchange basis on eBay? Worth it?
Or £60 for ones that look very similar but don't say they are for the P38.

kermit_rr · Sep 25, 2022

I think there is an alternative, that fits, possibly with an adapter. I'm not sure. They only last a few years so it's unlikely to be original.
I think there's a few places in the system that can leak pressure, there's a non return valve somewhere but not sure where.
It should hold pressure easily overnight so maybe you have more than one issue, i have a similar problem that I've not fixed (other more fundamental problems) where system losses pressure in under an hour

marjon · Sep 25, 2022

The same system was used on most Jaguar cars of that era too.
If they are cheaper or easy to get I dont know. But widens the search a bit.

J

Montyjohn · Sep 25, 2022

marjon said:
The same system was used on most Jaguar cars of that era too.
If they are cheaper or easy to get I dont know. But widens the search a bit.

J

Apparently Saab's too, but searches for them lead to forums advising owners to buy P48 units as they are much cheaper. Doh!
Jag units seem to be a similar price.

New or recon appears to be what it now boils down to.

pwood999 · Sep 25, 2022

Go for new. You don't know how good a "recon" might be, and as they are pressurized to 80bar or 1100psi, better to go new.

kermit_rr · Sep 25, 2022

I didn't know it was possible to refill them.. surely the rubber perishes and can't be replaced!?

marjon · Sep 25, 2022

I"m with @pwood999 Not sure how they can be reconditioned.
And is £200 really alot to pay to make sure the brakes work

.

J

aj1954 · Sep 25, 2022

£126.99 +vat from Island 4x4.

Montyjohn · Sep 25, 2022

marjon said:
I"m with @pwood999 Not sure how they can be reconditioned.
And is £200 really alot to pay to make sure the brakes work.

J

Apparently they pressure test them and refill the ones that pass the test, then run them through 100 duty cycles to check they are ok.
Whether 101 cycles would go pop is anyone's guess.
I don't know if they typically fail due to simply loosing gas or failing diaphragm.
If the former a recon could be a reasonable option.
My main hesitation on a new unit is I don't know that my accumulator is definitely at fault.

Montyjohn · Sep 25, 2022

aj1954 said:
£126.99 +vat from Island 4x4.

At £152 and free postage they do seem to be the cheapest available. Will probably go down this route thanks.

marjon · Sep 25, 2022

I would say it is from your description.
Have a read of this, he has a sense of humor.
Ho to check and replace Brake Accumulator in your Jaguar XJS (xjsbanger.co.uk)

J

kermit_rr · Sep 25, 2022

The rubber diaphragm is not a perfect seal and over time the nitrogen passes through it. In the same way balloons go flat

brianp38dse · Sep 25, 2022

So as the nitrogen passes through the diaphragm a full bleed of the brakes would not go a miss once new one is fitted.

Datatek · Sep 25, 2022

From your description, the accumulator is definitely faulty. Before trying to unscrew the accumulator, the brake pedal should be pressed 30 times.

kermit_rr · Sep 25, 2022

Does the pump run every so often without touching the pedal?

Grrrrrr · Sep 27, 2022

I thought @Mark Piercy found cheaper ones but maybe just in France?

Montyjohn · Sep 27, 2022

kermit_rr said:
Does the pump run every so often without touching the pedal?

I actually didn't know as I don't notice it when driving, so just stood checked with just the ignition on, and yes.
Every 4 minutes maybe it runs for less than a second.
Not timed.
So I'm guessing there's a leak somewhere.
My new accumulator arrived today. Do we reckon it's still worth swapping as part of the elimination process?

Where is the one way valve?

Montyjohn · Sep 27, 2022

Datatek said:
From your description, the accumulator is definitely faulty. Before trying to unscrew the accumulator, the brake pedal should be pressed 30 times.

Is it worth clamping the the hose from the reservoir or does fluid not run through after removing the accumulator?

P38A Why brakes don't seem right

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