Brake servo

[niveketak]

Having had no servo for 2 years and finding the brakes quite adequate if not awe inspiring, I had a close encounter with a family of deer on a road in the middle of nowhere and only just managed to stop having increased my thigh muscle size by 50 percent and altered the angle of my knee.

I decided to install one of Simons after market kits and having had to buy a brake pipe kit due to ends being different and the French not understanding that I would like to buy a pipe with a male and female end on, I today finished the installation and took Ruby out to try the brakes. Impressive is not the word, effortless braking with no difficulty locking all four wheels under emergency braking (fronts first followed by rears.)

Installation was simple with clear instructions and I cannot recommend the kit highly enough. I had to sort out two brake pipes in the end as the one that came with the kit was too long in any case but that was that. So if you don't have a servo and have not had to panic brake I would fit one just in case.

[Carlsson 986]

How much difference would a servo overhaul kit make to the braking rather than a new one?

My brakes are bordering on adequate and could certainly do with gioving some reassurance.

[swedishandgerman]

I have to say this - not because I don't think servos are brilliant because they REALLY are - but there is often a misconception of what servos do which is this:they decrease the amount of brake pressure exerted from the brake pedal to give an equal amount of braking performance.

In other words, the servo does not make the brakes work better. Instead, you just have to press the brake pedal less to get the same effect down at the wheels.

Another thing is this: these old Volvos had fantastic brakes for their era. Few other cars had big discs at the front with calipers with 3 pistons each as well as big drums with brake shoes with much bigger friction surfaces than most other similarly aged cars. There's a load compensator at the back and the system is easy to set up properly. Did you know that one of the best upgrades to the brakes on a Jaguar E-Type is to whack on a set of Amazon calipers!

Therein lies a problem. I would say that a large percentage of Amazons and 1800's do not have their brakes in tip-top condition resulting in, not poor, but adequate braking performance.

For example, I have seen on my Amazons them succesfully passing an MOT then to find at the next service at least one of the caliper pistons seized. So it will give a proper braking performance sufficient to pass the MOT on just 2 properly functioning pistons! That is leaving aside possible problems with swollen flexible brake pipes, poorly adjusted rear brakes, stretched handbrake cables making the shoes not returning properly, old brake fluid, ageing master cylinder seals and so on.

What I'm saying is that an Amazon with everything in proper condition should have pretty good brakes! That is brakes that you doon't have to stand on the pedal to get the thing to stop well. Prior to going down the route of getting a srvo, I would most definitely pay particular what's already there, starting with the fact that the old fashioned rubber seals in the front calipers go brittle quickly resulting in slow piston operation and at worst, no piston operation at all - both make you have to stand on that there brake pedal to get the old car to stop.

The thing with fitting a servo results in you inadvertently thinking you have great brakes even though half of your caliper is inoperative! Going back to my first comment that they decrease the amount of pedal pressure to give an equal amount of braking performance - if the brakes aren't completely up to scratch to begine with...

As I say, I'm a big fan of brake servos and have a very powerful one fitted to my estate (more powerful than the Brookhouse one) which requires very little 'push' on the pedal.

It reminds me of my BMW days when the 8-Series was a great car with HUGE brakes, but a servo with very little boost. They still stopped well, but you had to stand on the pedal. You could put a 7-Series servo in them making them a whole lot easier to drive.

[niveketak]

Swedish and German is perfectly correct in what he says, and I pride myself having fitted new callipers, pads, drums, shoes, wheel cylinders and adjusters along with the master cylinder, that my brakes were as good as they could be for non servo assistance and did lock the wheels if hit hard which is as much as a brake can do. However, my wife driving, yes she is allowed every now and again, did "feel" the brakes were "not working" as she is not used to non servo brakes. Being a bloke I didn't do anything as I knew they were good non servo brakes. However, being confronted with the deer family at 60 odd miles per hour, I recognised that an ordinary push on the pedal would not be enough delaying by a miniscule amount the emergency braking.

I have to say as well, for a reason I can't quite fathom, the car pulls up much straighter with the servo than before under heavy braking. Next step may be flexi replacement in case there is some light restriction in those overcome by the quicker fluid movement caused by the servo, which would confirm Swedish and Germans comments.

If I were you Carlsson I would take the Volvo somewhere quiet, hit the brakes hard at 30 mph or so and see if they all lock up. If they don't there is something awry and a good overhaul may be needed. In my experience as a Collision Investigator and vehicle examiner, assuming the servo is not contaminated with leaking brake fluid from the master cylinder and is not leaking vacuum past a seal which you would notice as the tick over speed would drop due to the constant loss of vacuum, then the servo will work. I cannot ever remember coming across a non working one in 8 to 10 thousand vehicle examinations.


I have a friend with a Morgan Plus 8 with no servo and that is scary by comparison to the non servo version of Ruby. His Morris Minor however, has non servo brakes that will stop on a sixpence (5p I suppose now being a similar size).

I can remember owning a 340 Volvo years ago and they were definitely over braked, probably too big a servo, as a light tap on the pedal would have your nose against the windscreen without intervention from the seat belt.

[swedishandgerman]

That's an odd thing and I've found that too - they pull up straighter. Don't know why! Maybe the assistance from the booster results in a better 'shove' of fluid down the pipes more forcefully.

There's one thing I'll add. I've read feedback on VW Aircooled stuff about peoples' responses to fitting more powerful remote servos as I have done about a kind of delayed reaction. I can't remember the actual boost that I went for, but it was much more than the standard Girling servo (1.1:1 I think) and more than the Brookhouse one (about 1.4:1) - I think mine is a 1.6:1 - but if you gently apply the brakes and/or gently come off them, there is a definitely noticable delayed reaction. Particularly when you come off the brakes - they stay on for a tiny but.

In retropsect, although it has an extremely light brake pedal, I wouldn't go for such a powerful one again for this reason.

[Derek UK]

The ratio is down to the diaphragm or pot size. Small servos can be 5 1/2" with 6" and 7" the next sizes. The vacuum will give a different "push to the plunger in the servo. Possible that the larger servo has a larger diameter plunger that will move a larger amount of fluid for the same linear movement, hence sharper brakes. Delayed action? This might also be down to the plunger movement being small for a large amount of action. Would seem to be counter intuitive though. In an emergency I would rather have a small servo than a large one as you're likely to have better control with the small one. Banging the brakes on with a big servo is likely to have you sliding into what you're trying to avoid.........

[niveketak]

The Brookhouse one is 1.9:1 so has a fair old heave on the brakes. I haven't been out much since I installed it as I have a shoulder problem which is stopping me driving for a week or so and as a result I can't comment on day to day usage as yet.

[222s]

Presumably it would be most appropriate when choosing which ratio Lockheed servo to use, to match the ratio of the original factory fitted Girling units? I've looked in the green books, but if that info was in there, I didn't spot it.