Brake Symptoms

[Thomas M]

Pumping the brake pedal frequently, say 3 or 4 times in 5 seconds, when coming to a stop from high or low speed, I notice that the pedal gets harder to depress, seems to have less stroke and the braking effectiveness diminishes quite a lot with each pedal push. It seems that if I then lay off of the brake pedal for a while and continue driving, everything returns to MY normal brake pedal feel and effectiveness.
Here are pics of the pump type, brake booster and vacuum output holding 29 inches. It holds for a while after shutting off the engine.
The vacuum line is good. No fluid leaks on or around the booster/master. Brake proportioning/equalizer near left wheel is dry.

If I let the car sit, engine off, for a while and then, with the engine still off, pump the pedal with my hand, it does the same thing....reduced stroke and hard feel. I hear a little hissing near the booster mounting as I pump it with my hand.



I haven't bled the brakes for some time, but I thought that would just cause a "mushy" feel to the brake pedal. The pedal doesn't go "to the floor".

Comments, please.

ftm1776

[TylerDurden]

I would check the vacuum as you repeatedly use the pedal (while engine running)... see if it drops. It may be booster time. I've put one in almost every MK1, MK2 I've owned.

If it happens even when testing at standstill, it is probably not fluid boiling or moisture ingress.

[82vdub]

If your engine has a lot of blowby, the output of the vane style pump pushes the air into the crankcase to create the vacuum. If you have high block pressures, this can deminish the braking system over several attempts at moving the pedal. Do you have a lot of air coming out of the valve cover if you pull the oil fill cap while it's running?

[Quantum-man]

Pumping the brake pedal frequently, say 3 or 4 times in 5 seconds, when coming to a stop from high or low speed, I notice that the pedal gets harder to depress, seems to have less stroke and the braking effectiveness diminishes quite a lot with each pedal push. It seems that if I then lay off of the brake pedal for a while and continue driving, everything returns to MY normal brake pedal feel and effectiveness.
Here are pics of the pump type, brake booster and vacuum output holding 29 inches. It holds for a while after shutting off the engine.
The vacuum line is good. No fluid leaks on or around the booster/master. Brake proportioning/equalizer near left wheel is dry.

If I let the car sit, engine off, for a while and then, with the engine still off, pump the pedal with my hand, it does the same thing....reduced stroke and hard feel. I hear a little hissing near the booster mounting as I pump it with my hand.



I haven't bled the brakes for some time, but I thought that would just cause a "mushy" feel to the brake pedal. The pedal doesn't go "to the floor".

Comments, please.

Hi,
1) Vacuum should last overnight. If it dissipates after say an hour, then you have a leak in the vac line, or brake booster somewhere.

2) If with the engine off, the pedal gradually stiffens with lesser and lesser travel, then the good news , is that your brake lines etc are good, as what I have described is normal. If bouncy, then there is air in the system.

3) Quickly running out of steam when going along implied in the case of my car, that the sump oil level was low, as the pump is centrifugal but oil pressure assisted. This is needed because the vanes are very light and need to be helped with oil behind them.

4) reading nearly 30 is good and implies that vanes are sealing reasonably.

5) Check sump oil level, open up pump, and ensure that vanes are free to slide out to the wall. Check that the gauze filter isn't blocked.
You may check leading edge of each vane.
If chipped, any of the other 3 leading edges can be substituted, thus essentially giving the pump endless life.

[Thomas M]

I would check the vacuum as you repeatedly use the pedal (while engine running)... see if it drops.

Tyler,

Idling, I get about 29 inches of vacuum. Pumping the pedal 5-6 times progressively with engine running reduces the vacuum to about 5 inches(correction). It returns to 29 inches in about 30(correction) seconds.

I removed the oil cap to try to reduce crankcase pressure....results were the same as above.

Trouble??? Does it sound like a new booster is on the list?????

Thanks, Thomas

[Thomas M]

If your engine has a lot of blowby, the output of the vane style pump pushes the air into the crankcase to create the vacuum. If you have high block pressures, this can deminish the braking system over several attempts at moving the pedal. Do you have a lot of air coming out of the valve cover if you pull the oil fill cap while it's running?

OK. So the vacuum pump pulls air from the booster and empties it into the crankcase. The greater the pressure in the crankcase, as if the combustion gas is leaking past the rings ("blow-by"), the less effective the vacuum pump action will be at delivering vacuum to the brake booster. Is this, more or less, correct???

I checked the output of the oil fill opening with the engine running. I can't tell if it is excessive or not, but there is a stream of oily air rising. Should there be some gas exhausting from the fill port???? Can you give some sort of a more graphic description as to what I am looking for??? What is "a lot"???? Should there be no blowby???? 290,000 miles on the beasty.

I had a diesel that apparently had so much blowby that the engine would "run away" at freeway speeds and bellow a nice black cloud to the rear!!! Is that where I am headed???? I recently had an overtemp situation that caused my heater core to explode and the car had to be "stop-cool-go" limped home for about 50 miles.

Latest mileage a week ago was 46mpg(US gallons); estimated oil consumption based on 1000 miles is 1/2 quart(US measure).

[Fatmobile]

Any idea what your oil pressure is?
Like Quantum_man mentioned; Those vane pumps are effected by oil pressure.

[bscutt]

I found that when I replaced my master cylinder that the seal in the booster was wet and dirty so I cleaned that before reinstalling the new MC. Vacuum has been much better since then, leading me to believe it was leaking around the pushrod seal. Might be worth checking that area and you can sometimes pull the MC forward without disconnecting everything.

[Thomas M]

Any idea what your oil pressure is?
Like Quantum_man mentioned; Those vane pumps are effected by oil pressure.

NO. Been meaning to install a gauge soon.

What is the normal oil pressure for the 1991 MK2 1.6 non-turbo engine???

Would an 80 psi gauge do the trick????

[Thomas M]

I found that when I replaced my master cylinder that the seal in the booster was wet and dirty so I cleaned that before reinstalling the new MC. Vacuum has been much better since then, leading me to believe it was leaking around the pushrod seal. Might be worth checking that area and you can sometimes pull the MC forward without disconnecting everything.

Good tip. I'll check that should I have to take the booster out.

[TylerDurden]

I checked my 86TD... pretty much the same behavior, but it might take 7-8 pumps on the pedal to get a sense the pedal was harder to press.

Vac reading 25-30" hg dropping to ~5" hg after six fast presses.

[surfcam]

I had problems with my stepvan vacuum asist brakes. My guess is that they work much the same. The problem with the VW throw away unit is you don't know what pressure is on the firewall side of the booster is. The vacuum has to be released from this side for the asist to work when brakes are applied and vacuum sealed in on the engine side. I've been tempted to drill a hole on the firewall side of the booster but don't know if I could seal it up again.

[vwtyp133]

Could also be a bad one-way valve to the booster. Underhood heat, oil fumes & plastics often don't play well together for extended periods.

[Thomas M]

Tyler,
Sounds pretty much the same as mine.......not going to worry about it.

Pretty hard to tell if the one-way valve near the vacuum pump or the booster diaphragm might be leaking........I think I'm ok for now.

Thanks

[bc]

Sorry to jack this thread, but are these types of vacuum pumps pictured here re-buildable? I know mine is shot.