Thread: Vacuum pump specs

What's the problem?

Have an ongoing brake problem. Need to know what the low and high vacuum that a 300Tdi vacuum should be generating. Mainly the maximum though.

Thanks,
Tom.

All I can tell you is that the vacuum at idle takes a good hold of your thumb.

Yep, it does that. Long running story, but I suspect that there may be too much vacuum. I now have reasonable pedal and sort of reasonable brakes after making my own tool to do a ABS modulator and power bleed on a 96 Disco. If I let it have full vacuum, pedal can get a little spongy and sink. And before you ask, car has been to two mechanics (one a recognised LR service centre). They had no idea and cost me $800 - I would not take a lawn mower there. Second mechanic and head TAFE teacher say it might be too much vacuum. I have been playing with a temporary hose trying let some vacuum leak and do seem to be getting some where. Hence the reason for wanting to know the maximum vacuum at which a Tdi300 Disco should be operating at. Might tray another bleed and go again in the morning.

Cheers,
Tom.

I don't know if there is any regulation of the vacuum beyond the vacuum pump ie in the brake booster. I've taken one vacuum pump apart to see how it works - just a couple of poppet valves with piston pump on a valve stem driven by the cam. My guess is there is a limit to the vacuum it can make - once it makes "full" vacuum there's nothing much left to evacuate - in which case I guess the poppets stop functioning. I've not seen any oil in the line that goes from the vacuum pump to the booster - oil travelling against the vacuum effect would not make sense - if I ever did see oil I would guess poppet valve failure. This is just my conjecture.

I was tempted earlier to ask if you have ABS. One of my vehicles has it. Plenty of threads about how to bleed it - mostly it seems, unsuccessful - and how to disconnect. I've disconnected it, as many times it has actually denied me power assisted braking on the first 20 metres or so of motoring. The only improvement - ABS connected or not - was cracking all the ABS lines one at a time and having someone in the cab pumping in the usual brake bleed manner. Not rock hard, but way better. The company that makes the ABS also makes your delightful vacuum pump (if original)!

And your pump is not leaking oil out of the welch plugs or between the head and main body?

Your master cylinder not leaking oil? Also, there are two seals in which your reservoir sits - the back one turns into a thin membrane bag that ruptures, and may allow some loss of pressure in the pump as it allows fluid to flow in and out of the cast iron cavity in which it is housed - does usually, but not always, let you know by a leak.

PS my other Discos are non-ABS and give no real brake problems.

How are the flexible hoses from chassis to brake calipers and from chassis to rear axle?
If all or some of them are weak in the flex reinforcement they can swell when high system pressure is applied and give that sinking feeling.
But it may be that, as others have said, it is a bit of air somewhere in the system and it does not take much air to give that spongy pedal.
I have never heard of too much vacuum in a brake system, as even engine generated (petrol engine manifold) vac systems are very variable up and down depending on throttle position, accelerating decelerating etc.
Know how you feel about the "experts", they play while you pay. Gaz

Originally Posted by gazby

How are the flexible hoses from chassis to brake calipers and from chassis to rear axle?
If all or some of them are weak in the flex reinforcement they can swell when high system pressure is applied and give that sinking feeling.
But it may be that, as others have said, it is a bit of air somewhere in the system and it does not take much air to give that spongy pedal.
I have never heard of too much vacuum in a brake system, as even engine generated (petrol engine manifold) vac systems are very variable up and down depending on throttle position, accelerating decelerating etc.
Know how you feel about the "experts", they play while you pay. Gaz

Hi Gaz, hoses are fine, wheel bearings are fine, new master cylinder and servo. The suggestion of too much vacuum came from the ex head TAFE automotive teacher and my mechanic. They both say that they have seen it do similar things before. I will do some power and modulator bleeds again in the morning. Pedal comes up hard with no vacuum. Pedal seems to be ok with an introduced leak in the system.

Will let you know how I go,.

Tom.

Another thing to look out for. Someone once posted a vid of the casing of a cheap after market booster expanding and contracting significantly when brake pedal cycled on and off.