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Bleeding (bloody) brakes


Indiana_Jones
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Hi all,

Just thought I might pick some brains here, currently trying to sort out the brakes on the XJ40, which the VCM is telling me has low pressure and the pedal does sink down a bit.

I've rebuilt the master cylinder (which the bore of looked to be in fairly good nick to me) as well as replaced the front flexi-lines. I've bled the brakes a couple of times, using the 2 man method, but for some reason, I still getting the low pressure.

I've given the pedal on over night a whirl, but no joy. Given that I'm getting low pressure at the start and not after multiple applications of the brake, I would imagine that the funky hydraulic booster system (with the accumulator etc) isn't causing the issue. 

Any other ideas?

Cheers

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Assuming the two man method is the British car method, pedal down close bleeder, pedal up open again? I think some models had some tie up with the ride height hydraulics? Cannot remember, another thing although unlikely is if there is adjustment possible on the pedal that someone has fucked with.

I admit I've given up a few times on some British stuff years ago and had them vacuum bleed at local specialist.

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Thanks for the input chaps. 

Yeah the method being done is where one person is at the brakes, the second person pumps up the brakes, holds the pressure. Then the person at the brakes opens the nipple, with the second person holding the pedal down. The first person closes the nipple off and then the second person removes the pressure on the pedal. 

I did give the MC a bleed before I proceeded onto the main system, but I'll double check. Also I saw Dicksmith had a vacuum bleeder on special, looked like a cheapie job, but given the price it might be worth a punt. 

Taistorm, yeah that's something else that had come into my mind, like the ABS system, but can't for the life of me figure out how one would bleed that (not in my workshop manual). 

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Not wanting to be a smart arse or anything but I've always bled brakes on my own using a rubber tube, blocked at one end with a slit cut in the side.

Just keep pumping away at the pedal keeping an eye on the reservoir, it's always seemed to work for me.

If I need to come up with a theory why it works I'd probably say you're forcing the air out, not trying to trick it out ?

I can't imagine I've just been jammy over the years.

/two penneth...

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1 hour ago, Indiana_Jones said:

Also I saw Dicksmith had a vacuum bleeder on special, looked like a cheapie job, but given the price it might be worth a punt. 

Dicksmith one is probably the same as ones from aliexpress? Which was the same ones resold on trademe, which could be cheaper, worth a look. 

I got one from Ali like this: https://www.trademe.co.nz/motors/car-parts-accessories/tools-repair-kits/other/listing-2613493571.htm?rsqid=954ab67877364b37be4e457500fcd1d6-001 (requires air compressor)

Haven't used it on brakes yet but it worked really well for draining my power steering!

I very rarely find someone willing to pump the pedal so just run some pipe into a bottle and open the bleed screw a little bit. Seems to work, once there is some fluid in the bottle, air can't get back up the hose if it's submerged. 

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22 hours ago, Indiana_Jones said:

Thanks for the input chaps. 

Yeah the method being done is where one person is at the brakes, the second person pumps up the brakes, holds the pressure. Then the person at the brakes opens the nipple, with the second person holding the pedal down. The first person closes the nipple off and then the second person removes the pressure on the pedal. 

I did give the MC a bleed before I proceeded onto the main system, but I'll double check. Also I saw Dicksmith had a vacuum bleeder on special, looked like a cheapie job, but given the price it might be worth a punt. 

Taistorm, yeah that's something else that had come into my mind, like the ABS system, but can't for the life of me figure out how one would bleed that (not in my workshop manual). 

The bleeding process you're describing is called 'surge bleeding'. It has it's uses but is prone to aerate  the fluid, particularly if there is a considerable amount of air in the system (e.g. after replacing a major component) and far more likely to occur in an ABS system (there are a lot of small valves/orifices in there). Aerated brake fluid has had air dissolved in it under pressure (as opposed to air bubbles) and  is your 'bleeding enemy'  (apologies for the bad pun!). 

 

I'm in favor of a more gentle approach; release the bleed nipple with the pedal in the rest position (i.e. compensation port uncovered), have the 2nd man (or the Mrs) smoothly push the pedal to the floor, tighten the bleed nipple, release the pedal and allow to rest for a few seconds - repeat until fluid is clear of bubbles. The air should come out in bubbles, if the fluid looks frothy of milky you're aerating it and things will only get worse. Never re-use fluid when bleeding even if it is new and clean, always allow it to rest for a few days - it takes quite some time for the dissolved air to separate.

I'm by no means an XJ40 expert (years since I've worked on one) but don't get sidetracked by the hydraulic booster, it only provides mechanical assistance to the pedal and is not connected to the hydraulics in the braking system.  In essence it does the same job as a vacuum suspended mastervac.

Hope this helps.

 

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Just now, Indiana_Jones said:

Thanks for that sr2, appreciated, will give that method a go the next time I get a chance. Also glad to hear that it should be separate to the booster system, as you do see mention of it, but by my logic that shouldn't be so. 

The modern 'Mastervac' style of booster (the XJ40 Hydraulic system is just a weird , overly complicated version of the same principal) is deliberately designed to be separate from the  brake systems hydraulics to avoid complete loss of brakes in the event of booster failure. Its predecessor the 'Hydrovac' is easiest described as a 'hydraulic amplifier' that was plumbed in series with the master-cylinder, in the event of its failure (which wasn't unheard of) the result was a complete loss of brakes.

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