Scary brake boost behavior

Hello again! This is the time of year where I get back to resolving issues instead of wandering about in the camper

Got a very scary behavior the other day while backing out of a tight spot in a public land boondocking site here in Quebec. So going in reverse, very very slow, in a slight incline to the rear, so I had my foot on the brake pedal all the time and was just ever so slightly pushing on and off the brake, repeatedly. At some point, I needed to stop faster and when I pressed down on the brake, it became REALLY hard and was only able to get to a slowwwww stop. Scary.

I have to say that in regular driving, my brakes work just fine.

Read up a bit on the forum and test the vacuum booster and master cylinder. With the engine off, after a few pumps, the pedal becomes hard, and then doesn't go down with sustained pressure. If I keep my foot on the pedal and then start the engine, it then goes down as vacuum builds up.

So I decided to give it a try in normal driving forward. At around 50mph, if I pump the pedal a good 3-4 times, on the 4th press of the pedal, it becomes really stiff again and in no way possible I could do a quick stop.

Can any of you report this as being normal?

Thanks for your always helpfull insight

From the service manual: (section 5, page 17)
MASTER CYLINDER
DIAGNOSIS AND TESTING - MASTER CYLINDER/ POWER BOOSTER
(1) Start engine and check booster vacuum hose connections. A hissing noise indicates vacuum leak.
Correct any vacuum leak before proceeding.
(2) Stop engine and shift transmission into Neutral.
(3) Pump brake pedal until all vacuum reserve in booster is depleted.
(4) Press and hold brake pedal under light foot pressure. The pedal should hold firm, if the pedal falls away master cylinder is faulty (internal leakage).
(5) Start engine and note pedal action. It should fall away slightly under light foot pressure then hold firm.
If no pedal action is discernible, power booster, vacuum supply, or vacuum check valve is faulty. Proceed to the POWER BOOSTER VACUUM TEST.
(6) If the POWER BOOSTER VACUUM TEST passes, rebuild booster vacuum reserve as follows:
Release brake pedal. Increase engine speed to 1500 rpm, close the throttle and immediately turn off ignition to stop engine.
(7) Wait a minimum of 90 seconds and try brake action again. Booster should provide two or more vacuum assisted pedal applications. If vacuum assist is not provided, booster is faulty.

...and then the POWER BOOSTER VACUUM TEST is in the paragraphs that follow.

Note the word "slightly" in step (5)

Service manual here: http://diysprinter.co.uk/reference/2006-VA-SM.pdf

--dick

Yep. That's pretty much what I did, and it comes out to a booster and master cylinder that work normally.
Has anyone else seen this behavior on their T1N?


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I have had this problem on another diesel vacuum assisted brake system (Eurovan). It is running out of vac. Why? From what I understand, like the Euro, vacuum on the sprinter is also used to open/shut flaps and valves in the HVAC. A small leak or bad valve will bleed off some vac and reduce the amount of vac available, especially at low rims

Patrick of M said:

I have had this problem on another diesel vacuum assisted brake system (Eurovan). It is running out of vac. Why? From what I understand, like the Euro, vacuum on the sprinter is also used to open/shut flaps and valves in the HVAC. A small leak or bad valve will bleed off some vac and reduce the amount of vac available, especially at low rims

Click to expand...

Oh darn..: that means i need to do a complete vac line debug...

Its just a pain to find vacuum leaks... a lot easier to find pressure leaks...

Thanks!


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Maybe try applying "mouth vac" to the vac supply line first and see if vac holds. With my eurovan the leak was in the reincarnation valve for cabin heat, as long as I wasn't in that position it was fine. The other solution is to add another vac reservoir so you have more leeway, that is a crappy solution masking a problem (but it works).

Did the traction control light on the dash flash? Did the pedal pulse? If so, you may have had all 4 wheels slipping, and the ABS was doing its thing.

That vacuum pump is rather small and I will bet, doesn't move a lot of air. It sounds like you were using the brakes so much as to exhaust the vac bottle and the vac pump could not keep up. Probably a very rare situation. Check all of the vac connections and if you don't find anything keep a close feel on the operation of the brakes. I will bet it will not return unless you get in to this situation again.

Charles

I have seen some vehicles where a reservoir has been added to a vacuum system to create additional reserve volume (usually short time duration though).

That said, the braking issue under discussion hasn't been a hot topic here on the forum. As has been suggested, there may be a leak somewhere or other system issue.

vic

Midwestdrifter said:

Did the traction control light on the dash flash? Did the pedal pulse? If so, you may have had all 4 wheels slipping, and the ABS was doing its thing.

Click to expand...

Nope not at all. And terrain was good enough not to allow slippage at that speed


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I actually occasionally have this same issue while backing up my trailer with my T1N. It might unfortunately be a normal response of running out of vacuum at idle. A little throttle and the problem is resolved.

had a thought , if the vac runs the ac , have you tried shutting everything else down ? See if the changes how the breaks react

The turbo and a/c recirculation flap are vacuum powered on pre-2004 models, but fully electric for 2004 and later, so debugging the vacuum plumbing on a 2006 should be straightforward.

The vacuum pump’s displacement is relatively small, so at idle speed it does not keep up to repeatedly applying and releasing the brake pedal. A vac leak would exacerbate this, but as the test above indicates the reservoir is good for only three or four full pumps of the pedal with the engine off, and it’s easy to pump the pedal at idle and exhaust the vacuum within (seven or eight?) pumps. I simply shift to neutral and rev the engine briefly on the rare occasion that I feel the pedal start to firm up.

Might be a useful poll: how many times can you pump your brakes at idle before noticing a loss of vacuum assistance?

-dave

Hello,
Im having a similar issue. Brakes were just done and I noticed vacuum pump leaking. I had oil all over. I rebuilt it... Well replaced all seals. The little paddle on the inside fell out. It didn't look like it mattered which direction it was in when I put it back. Also, the hose coming off pump to master cycilnder was a real headache getting off. Passed all the tests. Then I noticed the same brake issue you described above. It only happens at idle when I'm really trying to finesse a tricky parking spot. I came to the conclusion that the little pump cant keep up.... Unless one of those two items I described above messed up my system. I feel like it has always been like this but im only payong attention to it now cuz i worked on it..... So yeah a poll would be great.

There may be some info here to help.


vic

If any of you have ever driven an older vehicle with air brakes often the compressor can't keep up either at idle when you are doing a lot of parking manoeuvres. the nice thing with air brakes is you have a dial on your dash and you can see that you're losing air and eventually you put on the brakes and rev it a bit until the pressure is back. It works the same with a vacuum pump, if you are losing vacuum put it in neutral and rev it a bit or put it in park and rev it a bit.
There is one other detail , the sprinter's vacuum is used to actuate the air vents in the cockpit and if lines, fittings or mechanisms are cracked or leaking you will use your vacuum faster/have trouble keeping the vacuum strong enough.

Problems of this nature are best diagnosed with a vacuum gauge. I think a vacuum gauge is a vital tool for any sprinter owner, if you are to can not afford to buy a good one you can get away with one out of the dash of an early model car where they were installed as an economy gauge.
If you use the vacuum gauge by connecting it to the "T" on the booster to get a reference reading then add your own "T" to monitor the system by disconnecting items one at a time till you find the leak. Eric.

I find using a mityvac lets me both monitor and test. May not be ideal for driving, but I've found issues i never would have without it.
ohh, you could mount it by the lumbar support as an emergency brake boost!

Luc, did you ever diagnose and fix this? I'm having the same issue, and the tests in the service manual pass on my van.

LoopM said:

Luc, did you ever diagnose and fix this? I'm having the same issue, and the tests in the service manual pass on my van.

Click to expand...

Though this is an old thread, your issue is current and should be addressed.
If you read this thread carefully and completely, the key to the original problem is this quote in the original post: "...I had my foot on the brake pedal all the time and was just ever so slightly pushing on and off the brake, repeatedly." That action bled off the vacuum from the booster and caused the hard pedal symptom. This is a completely normal consequence if the engine is idling and the booster is continually being bled by brake pedal action. If you were doing the same thing as Luc, then you simply need to be aware and do not pump the pedal while the engine is idling and moving.

This does not account for your situation if you have vacuum leaks in any of the lines that connect to the vents for the cabin, as mentioned, so be clear under what circumstances you experienced the hard brake pedal.