Sierra Brake Servo

[andyparry]

I've been using the original sierra estate brake servo and master cylinder since I built the car in 2003. When I took the pinto efi lump out and fitted the blackbird, I wanted to retain the servo as I don't like the feel of a servoless system. I therefore tapped off each of the 4 carb intakes, manifolded them together and used that as the vacuum source for the servo. The problem was that due to the small pipe size, it took a while to build up pressure, meaning I only had one good hit on the brakes before I was into the uncomfortable feel territory.

I've just fitted an electric vacuum pump from and x-type and linked directly to the servo. All is plumbed up and wired. If I put my finger over the pipe it draws a good vacuum and the pump pitchchanges. When I run it on the servo, there is no change of pitch, even when I press the pedal.

So my question is, how does the servo work? I would have expected the pump to evacuate the servo and hold it under a reduced pressure.....or do I have a knackered servo?

Andy

[Peter Bell]

Is there a pitch change when you press the pedal really hard? The vacuum is applied to both sides of the diaphragm at rest and modulated as the brakes are applied, there is an air bleed on the pedal side of the servo. But then again 30+ year old rubber seals won't be working at their best

[Bob Tucker]

Andy if it turns out the servo is fubared, I have one you can have. No charge.

[agent_zed]

Hey, i'm looking at converting to bike carbs myself and was going to do the 4 pipes to a small chamber with vacuum pipes off to the servo and dizzy. What size pipe did you use? Don't want to make the same mistake. Sorry don't have an answer on the electric pump.

[Peter Bell]

My servo runs from just one runner on a bike carb manifold and I've never noticed any problems with lack of assistance, I suspect the original problem is not so much the pipe size as the servo.

[Bob Tucker]

Andy Im off on hols (again!) today, but back on 16th if you need the servo just PM & Ill respond when I get back

[andyparry]

I finally found some time last night to take the car for a test drive to see if the new vacuum pump was working. All actually seem to be working really well.

 

Peter - your explanation on how the servo works is spot on. There is no pitch change when the brake pedal is untouched. There is a slight change when the pedal is pressed. Either way, the brakes are back to being a sharp and good feel as I would like them. This also suggests that my brake servo is in fact actually working as well.

 

Bob - thanks for the offer of the servo. Much appreciated, but now not needed.

 

Agent Zed - the 4off screw threads into my carbs are all 4mm. That's a root of 3.3mm. I therefore took some M4 screws and drilled a 2mm hole up the middle. That gave me a total area across all 4 carbs, of about 12.5mm^2. The new x-type pump has an input port of ~50mm^2, so I've got 4 times the air flow that I previously had.

 

The down side to all of this is that something else has now gone wrong! It seems that the engine has gone out of tune when idling or at low revs. It is very lumpy, almost as if it is running on 3 at low revs. I'm wondering if having the engine tuned with the 4 carbs all manifolded together, albeit through 3mm ID piping, and now removing that manifold has had an adverse effect on the tuning. I'm going to whip the carbs off and have a look to ensure that the new 4 blanking screws are not sticking out in to the carbs and affecting the airflow. After that, apart from the normal checks for sparks and fuel, I think it will be a trip to the tuners. Any other ideas?

 

Thanks for your comments chaps.

Andy