Odd brake vacuum issues

[Westwood]

This is my first post, so forgive any newbie mistakes. I searched for answers before registering and couldn't find any directly on point, hence this post.

1981 VW Diesel Rabbit with an extremely hard and almost ineffective brake pedal. Naturally, a rebuilt vacuum pump was in order. Local indy VW mechanic (35 years experience, primarily with gas VWs, but has had diesel experience as well) has several of the used diaphragm pumps available and several rebuild kits and has rebuilt them successfully many times in the past. When a rebuilt pump is installed (3 different ones, so far) it creates a vacuum for the brake booster (aka servo) but possibly only a weak one, hard to tell, really and I have no idea what the spec is in this regard.  To test a power brake system you turn off the engine, pump up the brake pedal, then start the engine. When the engine is started, the pedal will slowly sink. Test failed repeatedly after rebuilt pump installations. Pedal stays rock hard on start-up.

However, to see if the problem was with the vacuum pump(s) or with the vacuum booster or with the master cylinder we applied vacuum from a car running next to it with a long hose from the running car to the diesel's vaccum booster. (I.E. using a different, gas engined car, to generate vaccum instead of the rebuilt vacuum pump)  Test passed!

Does anyone have any idea what is going on here?  Your help is greatly appreciated and my apologies for an extremely long post.

[Op-Ivy]

Check your lines. There is a check valve in the line from the brake booster to the vacuum pump. Your brakes may do funny things if that is not working.

Also, check the line altogether. Try using a fresh line.

If you rev the engine does the pedal get softer at all?

[Possum79]

All i can say is brakes suck. Mine just went out on me the other day. Pedal down and very little affect. I gotta look at it this weekend.

Good luck on yours. :wink:

[Westwood]

We used a fresh line from vacuum pump to brake booster and also detached the other line that vents to the crankcase, in case that was blocked, so that it vented to the atmosphere (tried it both ways, with that second line detached and also attached).  Also we checked the check valve (mounted to the surface of the booster) by taking it out and blowing through it. It only allowed air in one direction, towards the pump as it should be. Plus it was that same valve that we attached the foreign external vacuum hose to, which worked.

Revving the engine had no effect on the pedal either.

[Op-Ivy]

That's really odd. If the problem was fixed using another form of vacuum, then the problem must have to do with the pump or the line. Try getting a vacuum gauge and hooking it up to see what you are getting for suction.



Matt

[denroldiesel]

This is my first post, so forgive any newbie mistakes. I searched for answers before registering and couldn't find any directly on point, hence this post.

1981 VW Diesel Rabbit with an extremely hard and almost ineffective brake pedal. Naturally, a rebuilt vacuum pump was in order. Local indy VW mechanic (35 years experience, primarily with gas VWs, but has had diesel experience as well) has several of the used diaphragm pumps available and several rebuild kits and has rebuilt them successfully many times in the past. When a rebuilt pump is installed (3 different ones, so far) it creates a vacuum for the brake booster (aka servo) but possibly only a weak one, hard to tell, really and I have no idea what the spec is in this regard.  To test a power brake system you turn off the engine, pump up the brake pedal, then start the engine. When the engine is started, the pedal will slowly sink. Test failed repeatedly after rebuilt pump installations. Pedal stays rock hard on start-up.

However, to see if the problem was with the vacuum pump(s) or with the vacuum booster or with the master cylinder we applied vacuum from a car running next to it with a long hose from the running car to the diesel's vaccum booster. (I.E. using a different, gas engined car, to generate vaccum instead of the rebuilt vacuum pump)  Test passed!

Does anyone have any idea what is going on here?  Your help is greatly appreciated and my apologies for an extremely long post.

There is a vacuum pump driven off the engine block.  If this is defective and lost its ability to make a vacuum then that is your cause.

[Op-Ivy]

This is my first post, so forgive any newbie mistakes. I searched for answers before registering and couldn't find any directly on point, hence this post.

1981 VW Diesel Rabbit with an extremely hard and almost ineffective brake pedal. Naturally, a rebuilt vacuum pump was in order. Local indy VW mechanic (35 years experience, primarily with gas VWs, but has had diesel experience as well) has several of the used diaphragm pumps available and several rebuild kits and has rebuilt them successfully many times in the past. When a rebuilt pump is installed (3 different ones, so far) it creates a vacuum for the brake booster (aka servo) but possibly only a weak one, hard to tell, really and I have no idea what the spec is in this regard.  To test a power brake system you turn off the engine, pump up the brake pedal, then start the engine. When the engine is started, the pedal will slowly sink. Test failed repeatedly after rebuilt pump installations. Pedal stays rock hard on start-up.

However, to see if the problem was with the vacuum pump(s) or with the vacuum booster or with the master cylinder we applied vacuum from a car running next to it with a long hose from the running car to the diesel's vaccum booster. (I.E. using a different, gas engined car, to generate vaccum instead of the rebuilt vacuum pump)  Test passed!

Does anyone have any idea what is going on here?  Your help is greatly appreciated and my apologies for an extremely long post.

There is a vacuum pump driven off the engine block.  If this is defective and lost its ability to make a vacuum then that is your cause.

Obviously you didn't even read the post you quoted. Westwood said he has replaced it a few times.

[denroldiesel]

This is my first post, so forgive any newbie mistakes. I searched for answers before registering and couldn't find any directly on point, hence this post.

1981 VW Diesel Rabbit with an extremely hard and almost ineffective brake pedal. Naturally, a rebuilt vacuum pump was in order. Local indy VW mechanic (35 years experience, primarily with gas VWs, but has had diesel experience as well) has several of the used diaphragm pumps available and several rebuild kits and has rebuilt them successfully many times in the past. When a rebuilt pump is installed (3 different ones, so far) it creates a vacuum for the brake booster (aka servo) but possibly only a weak one, hard to tell, really and I have no idea what the spec is in this regard.  To test a power brake system you turn off the engine, pump up the brake pedal, then start the engine. When the engine is started, the pedal will slowly sink. Test failed repeatedly after rebuilt pump installations. Pedal stays rock hard on start-up.

However, to see if the problem was with the vacuum pump(s) or with the vacuum booster or with the master cylinder we applied vacuum from a car running next to it with a long hose from the running car to the diesel's vaccum booster. (I.E. using a different, gas engined car, to generate vaccum instead of the rebuilt vacuum pump)  Test passed!

Does anyone have any idea what is going on here?  Your help is greatly appreciated and my apologies for an extremely long post.

There is a vacuum pump driven off the engine block.  If this is defective and lost its ability to make a vacuum then that is your cause.

Obviously you didn't even read the post you quoted. Westwood said he has replaced it a few times.

Yep..my bad .. I apologise. I will tell you why I made that error.  I go through a "fist principal critical path".  I assume nothing.

First item here was the pump.  Second item is the hose, integrity then material and size.  The last one we had like this was a 91 Golf gasser.  Same symptom. The rubber hose had persihed.  Sometimes folks use hoses with thin walls that collapse under vacuum pressure. Third item would be the vacuum unit.

[denroldiesel]

This is my first post, so forgive any newbie mistakes. I searched for answers before registering and couldn't find any directly on point, hence this post.

1981 VW Diesel Rabbit with an extremely hard and almost ineffective brake pedal. Naturally, a rebuilt vacuum pump was in order. Local indy VW mechanic (35 years experience, primarily with gas VWs, but has had diesel experience as well) has several of the used diaphragm pumps available and several rebuild kits and has rebuilt them successfully many times in the past. When a rebuilt pump is installed (3 different ones, so far) it creates a vacuum for the brake booster (aka servo) but possibly only a weak one, hard to tell, really and I have no idea what the spec is in this regard.  To test a power brake system you turn off the engine, pump up the brake pedal, then start the engine. When the engine is started, the pedal will slowly sink. Test failed repeatedly after rebuilt pump installations. Pedal stays rock hard on start-up.

However, to see if the problem was with the vacuum pump(s) or with the vacuum booster or with the master cylinder we applied vacuum from a car running next to it with a long hose from the running car to the diesel's vaccum booster. (I.E. using a different, gas engined car, to generate vaccum instead of the rebuilt vacuum pump)  Test passed!

Does anyone have any idea what is going on here?  Your help is greatly appreciated and my apologies for an extremely long post.

There is a vacuum pump driven off the engine block.  If this is defective and lost its ability to make a vacuum then that is your cause.

Obviously you didn't even read the post you quoted. Westwood said he has replaced it a few times.

Yep..my bad .. I apologise. I will tell you why I made that error.  I go through a "fist principal critical path".  I assume nothing.

First item here was the pump.  Second item is the hose, integrity then material and size.  The last one we had like this was a 91 Golf gasser.  Same symptom. The rubber hose had persihed.  Sometimes folks use hoses with thin walls that collapse under vacuum pressure. Third item would be the vacuum unit.

 :oops:

Just taken a second opinion on this fault.  Line check then servo seal suspect.  There is also a valve in the line, one way..best replaced.. looks like a flying saucer.  We have a 80hp 1,6TD here and we have gone to see.   We think that the valve with the yellow colour top on could be faulty.   With brake servos we take the seals out and wash em in thinners.