Odd braking issue

Odd braking issue

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mrtwisty

Original Poster:

3,038 posts

134 months

Saturday 10th July
quotequote all
Hello folks

2005 Dispatch, 1.9d (non-turbo)

I have an odd issue - the brakes are only assisted for one quick use, then the pedal goes hard. Oddly it's worse with a light application of the brakes - a hard application results in some decent brake force for a longer period (briefly!)

Vacuum assistance soon returns when you come off the brakes and give it some revs, but then gone again just as quickly the next time you use them.

I've only just bought the van and there appears to be fairly new vaccum pump (which I've had off and checked - fresh seals, new pipework, unworn cam pegs). 

What should I be looking at next? Servo?


Smint

184 posts

4 months

Saturday 10th July
quotequote all
Maybe run engine for a few moments to charge the vaccum up, then switch off somewhere deathy quiet anbd listen carefully for leaks, there should be enough vaccum held for two or three assisted operations of the brakes.
Maybe also check the new pump is the correct one for the engine and of suitable quality.


mrtwisty

Original Poster:

3,038 posts

134 months

Sunday 11th July
quotequote all
Listening for a vac leak - now that's a good idea, thanks!

I've just done a (careful!) 60 miles in it, and it's really quite driveable if you give the brakes a decent stab. It really is strange... you can feel something 'give' at about 1/3rd of the pedal travel, after which is pulls up just fine - any less than this though and it very quickly loses the servo assistance.

GreenV8S

28,975 posts

253 months

Sunday 11th July
quotequote all
I think your vacuum port isn't sealing properly. This is supposed to be sealed during the first part of the pedal travel to separate the two halves of the servo so that the air inlet valve can generate a differential pressure. If the vac valve doesn't seal, as soon as the air inlet valve opens all the vacuum dumps through it. This also fits the symptoms of the servo working OK under firm braking since there would be more pedal travel and more pressure trying to close the vacuum port.

The fix will be a new servo, since these are designed to be sealed units.

ETA: It's just possible that the master cylinder pushrod has been adjusted too long so the brake servo doesn't have enough travel to close the port before the brakes start resisting travel, but I don't know whether it's adjustable on that car. I guess you'll find out when you take the servo off to replace it.

Edited by GreenV8S on Sunday 11th July 15:47

mrtwisty

Original Poster:

3,038 posts

134 months

Thursday 15th July
quotequote all
GreenV8S said:
I think your vacuum port isn't sealing properly. This is supposed to be sealed during the first part of the pedal travel to separate the two halves of the servo so that the air inlet valve can generate a differential pressure. If the vac valve doesn't seal, as soon as the air inlet valve opens all the vacuum dumps through it. This also fits the symptoms of the servo working OK under firm braking since there would be more pedal travel and more pressure trying to close the vacuum port.

The fix will be a new servo, since these are designed to be sealed units.

ETA: It's just possible that the master cylinder pushrod has been adjusted too long so the brake servo doesn't have enough travel to close the port before the brakes start resisting travel, but I don't know whether it's adjustable on that car. I guess you'll find out when you take the servo off to replace it.

Edited by GreenV8S on Sunday 11th July 15:47
That sounds spot on, thanks for the detailed knowledge GV8S!

New servo it is then... looks like a bit of a pig to do, though I have seen a dirty bodge suggestion of cutting out a section of the bulkhead to get to it!