draper40 Posted July 29, 2009 Share Posted July 29, 2009 Hey, I've just rebuilt my master cylinder and tested my vacuum unit yesterday by pressing the brakes as firm as possible, then starting the egine and there was no more give. Do these units often fail and where are they likely to fail? Thanks Pete Quote Link to comment Share on other sites More sharing options...
LandyManLuke Posted July 29, 2009 Share Posted July 29, 2009 Are you getting vacuum from the pump? pull the pipe off the servo and stick your finger over the end, with the engine running. Quote Link to comment Share on other sites More sharing options...
darthdicky Posted August 5, 2009 Share Posted August 5, 2009 I've got a similar problem - I do get some vacuum assistance (and the pedal drops when the engine is started) but I have a feeling there should probably be more. I drove a TD5 ages ago that had a much sharper response to the pedal. I'm getting suction from the pump, is it likely that the servo itself needs replacing? Rich Quote Link to comment Share on other sites More sharing options...
errol209 Posted August 5, 2009 Share Posted August 5, 2009 I've got a similar problem - I do get some vacuum assistance (and the pedal drops when the engine is started) but I have a feeling there should probably be more. Same here, and I've been meaning to ask if someone with a diesel 90 and a very good servo response could describe how much vacuum theirs produces in terms of discomfort level (of thumb over end of pipe, of course)? I can easily remove said digit with engine at mid revs, suggesting my vacuum pump aint! Update - I've just located the pump pressure spec in the WSM, and it says 0.8 bar. I'm guessing this means it generates .2 bar of vacuum below atmosphere? Quote Link to comment Share on other sites More sharing options...
WESTENDER Posted August 6, 2009 Share Posted August 6, 2009 isnt bar a measurement of positive pressure ? ie atmospheric pressure = 14.7 Lbs = 1 bar so i am guessing 0.8 bar is the assisting pressure given by the servo? this seemingly low pressure would obviouly be amplified by the surface area of the servo diaphragm vacuum is measured in inches of mercury surely? as an ex fireman this was the case on our appliances Quote Link to comment Share on other sites More sharing options...
martinspragg Posted August 7, 2009 Share Posted August 7, 2009 I have had vacuum assistance problems and replaced the vacuum pump and (eventually) the servo to sort them out. The "discomfort" factor at idle on the vac hose is low i.e. your thumb will stick to the end but be easily taken off. Taking atmospheric pressure at 1bar or 14.7psi a pressure of 0.8 bar is equivalent to -ve 0.2 bar or -ve 2.94 psi. I guess the id of my vacuum pipe to be 3/8" equivalent to a cross sectional area of 0.11sq in and so at-ve 2.94psi you should get a "pull" of 0.32lbs or about 5ozs. Sorry about the imperial measurements but it is what I am familiar with, regardless it shows that the pump is not going to stick your thumb to the tube and give you a hickey! Getting back to the OP I think the servo should be investigated, that was my problem and it made a ton (a long one at that) of difference. My only small issue is that I think my idle is too low and so at parking speeds it is easy to use up all the vacuum and you have to wait for it to get build up again. Quote Link to comment Share on other sites More sharing options...
Les Henson Posted August 7, 2009 Share Posted August 7, 2009 The easiest way to test pump vaccuum is to drive the vehicle at a steady speed and brake repeatedly. If the braking performance fades, then the pump isn't producing enough vaccuum. There's a one-way valve on the end that plugs into the servo, which can fail - allowing vaccuum to go either way and producing weak brakes. You need to test the pump itself by removing the servo pipe, then the valve by trying to blow through it, then the servo itself for leaks - switch the engine off and leave it for 30-minutes, then pull the pipe out and it should hiss as the vaccuum is lost. Les. Quote Link to comment Share on other sites More sharing options...
errol209 Posted August 7, 2009 Share Posted August 7, 2009 The easiest way to test pump vaccuum is to drive the vehicle at a steady speed and brake repeatedly. If the braking performance fades, then the pump isn't producing enough vaccuum. There's a one-way valve on the end that plugs into the servo, which can fail - allowing vaccuum to go either way and producing weak brakes. You need to test the pump itself by removing the servo pipe, then the valve by trying to blow through it, then the servo itself for leaks - switch the engine off and leave it for 30-minutes, then pull the pipe out and it should hiss as the vaccuum is lost.Les. In the bath playing with my duck last night I hit on the idea of using my turbo boost gauge and suitable bodge adapter to measure the pump vacuum, as I don't seem to get any assistance at all and I need to start elimimating problems. I suspect my problem is the servo unit though. Good luck to all! Quote Link to comment Share on other sites More sharing options...
Paul H Posted August 7, 2009 Share Posted August 7, 2009 1 bar = 15psi approx 0.8 bar = 12 psi approx These are both positive pressure. Vacuum is negative pressure so i would expect that the pump spec is for -0.8 bar or -12psi vacuum. Quote Link to comment Share on other sites More sharing options...
Aragorn Posted August 7, 2009 Share Posted August 7, 2009 "1 bar" stated alone is no guarantee of positive pressure. Atmospheric pressure is approximately 1 bar in absolute terms. so 0.8 Bar in absolute terms is infact a vacuum, as its 0.2bar below atmospheric pressure... Quote Link to comment Share on other sites More sharing options...
errol209 Posted August 7, 2009 Share Posted August 7, 2009 "1 bar" stated alone is no guarantee of positive pressure.Atmospheric pressure is approximately 1 bar in absolute terms. so 0.8 Bar in absolute terms is infact a vacuum, as its 0.2bar below atmospheric pressure... That was my interpretation, because 0.8 bar or 9 psi below atmosphere over the area of the servo (94 square inches) gives about 854 pounds or 3.8kN at earth gravity! 0.2 bar or 2 psi gives about 214 pounds which is a shade more believable. That's what I love about this forum - 33% science, 33% empirical / experience and 34% humourous irrelevance! Quote Link to comment Share on other sites More sharing options...
errol209 Posted August 9, 2009 Share Posted August 9, 2009 That was my interpretation, because 0.8 bar or 9 psi below atmosphere over the area of the servo (94 square inches) gives about 854 pounds or 3.8kN at earth gravity! 0.2 bar or 2 psi gives about 214 pounds which is a shade more believable.That's what I love about this forum - 33% science, 33% empirical / experience and 34% humourous irrelevance! Update: I connect the boost gauge to the vacuum pump, and according the the gauge (its more or less accurate about the turbo boost pressure) I got a reading of -1 bar! Therefore the 0.8 bar vacuum is precisely that; 0.8 bar below atmospheric minimum. I nearly lost my socks when I got the non-retrun valve off, so the servo is holding vacuum and the non-return valve works fine: I'm guessing its the valve and inlet filter (which is good, 'cos the overhaul kit is only £10) More news shortly. p.s. the joke in the previous post should have read: "That's what I love about this forum - 35% empirical / experience and 35% humourous irrelevance and 35% science!" Quote Link to comment Share on other sites More sharing options...
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