Think I’ve broken my servo

[RichardHyde]

I bled the brakes after replacing one of the copper brake pipes and found that the pedal was rock solid. When I pushed harder, there was a cracking sound and the pedal went to the floor.

I’m assuming that I’ve broken the bakerlite in the servo ?

I’m aware that Sprint servos are hard to find. Is it possible to replace the Bakelite ?

Are other Dolomite servos the same dimension (except circumference) ?

Thanks, Richard

[xvivalve]

Sprint servos are plentiful, I have a box full...but they all have a broken bakelite piston and it's that component which is obsolete.

Club spares currently have two reconditioned servos, but they are £200 plus a £100 exchange surcharge, so if your's is not serviceable I'm afraid it's a £300 bill. Other models of Dolomite have different sized servos both diameter, depth and boost ratio. The Sprint isn't quite unique as the same unit was used in some early Fords, but they have the same issue. Our folk are still engineering with light alloy and 3d printed solutions, but unfortunately are experiencing issues with both avenues of research.

The late club member Paul Wilbrahim created an adaptor that allowed the use of a Land Rover servo to be fitted. I have passed his research to our reconditioners along with his prototype, but upon testing the boost they report the Land Rover unit is significantly less than a Sprint unit, so at the moment they are leaving that idea on the shelf.

Your other option is to use an 1850 unit, but again, boost would be slightly reduced.

Late 1850's were factory fitted with Sprint units, so that's a potential source if you find one and an OE 1850 unit to swap out....

[shaunroche]

There's a lad at work here who's mad about 3d printing....if someone can supply a fag packet sketch I could ask him about getting some made perhaps.

[xvivalve]

There's a lad at work here who's mad about 3d printing....if someone can supply a fag packet sketch I could ask him about getting some made perhaps.

It's more complex a casting than a fag packet sketch (and also larger than standard printers can contend with); they're in discussion with the right people, but an issue they have is getting sufficiently smooth a surface. Also there is a crucial fine slot with a thin wall that is causing problems with both 3d and alloy, albeit different problems!

[shaunroche]

There's a lad at work here who's mad about 3d printing....if someone can supply a fag packet sketch I could ask him about getting some made perhaps.

It's more complex a casting than a fag packet sketch.....!

Which bit are we talking about on here?....



I've never seen inside a servo before!

[Carledo]

The Carledo has run an 1850 servo the last 7 years or so, it's Sprint servo gave up the ghost and I was unwilling to pay the exorbitant price then (or now come to that) for another. This was before club servos and Rimmers ones were available and it turns out the Rimmer ones are problematic.

So I thought i'd experiment and it works fine. The (early) 1850 servo is a tad smaller in diameter and gives a little less boost but it IS only a little less and I find with that car, light as it is, the pedal has more "feel" to it than with a Sprint one fitted, I actually prefer it!

The Vauxhall converted 1500FWD that is my current daily drive still has it's original 1500FWD servo (with TJs etc and a dual master) and that stops OK too (OK by MY standards is pretty good) I'm certainly not hankering for more boost.

Steve

[cleverusername]

I think the issue with 3d printing, at least in my experience, is 3d prints aren't particularly strong. They aren't going to be happy in a position in which they have to take load. To be fair I have only used PLA, so something like ABS might work better and I am using a consumer level printer.

[RichardHyde]

Does the servo ONLY give brake boost ?

Will the car run normally apart from that ? Is the car road legal with a faulty servo ?

[Tony Burd]

I would not drive the car any distance, accept maybe to get home, with a broken servo, the brakes are designed to rely on the servo boost to operate correctly. If you have ever been towed you will know you have to really stamp on the brakes to stop the car when the engine is not running.

A non working servo would be an MOT failure I would think, so yes illegal to drive.

[Carledo]

Does the servo ONLY give brake boost ?

Will the car run normally apart from that ? Is the car road legal with a faulty servo ?

All a servo does is reduce the pedal pressure required to work the brakes. But it can be anywhere from 1.25/1 to 2/1 boost ratio, I think the Sprint one is around 1.75/1, so the brakes will require near twice as much pedal pressure to achieve the same result as if the servo was working. No fun at all! A duff servo IS an MOT failure and very likely illegal too, under C+U regs.

I'm not entirely sure the servo is at fault here, a duff servo would not cause the pedal to go to the boards, quite the reverse, it would leave the pedal rock hard, right at the top, and doing nothing much to stop the car. My own intuition says the master cylinder is a more likely candidate for your fault.

Steve

[xvivalve]

Yes, it would leave the pedal rock hard until the pushrod shatters the piston, then the pedal would have potential for no resistance.

[new to this]

Sprint servos are plentiful, I have a box full...but they all have a broken bakelite piston and it's that component which is obsolete.

Club spares currently have two reconditioned servos, but they are £200 plus a £100 exchange surcharge, so if your's is not serviceable I'm afraid it's a £300 bill. Other models of Dolomite have different sized servos both diameter, depth and boost ratio. The Sprint isn't quite unique as the same unit was used in some early Fords, but they have the same issue. Our folk are still engineering with light alloy and 3d printed solutions, but unfortunately are experiencing issues with both avenues of research.

The late club member Paul Wilbrahim created an adaptor that allowed the use of a Land Rover servo to be fitted. I have passed his research to our reconditioners along with his prototype, but upon testing the boost they report the Land Rover unit is significantly less than a Sprint unit, so at the moment they are leaving that idea on the shelf.

Your other option is to use an 1850 unit, but again, boost would be slightly reduced.

Late 1850's were factory fitted with Sprint units, so that's a potential source if you find one and an OE 1850 unit to swap out....

Is the reason the baker light piston brakes ? old age making it brittle
Or a design fault, not strong enough in the first place

Is it a complicated shape making it expensive to have made machined ?

Dave

[Carledo]

Yes, it would leave the pedal rock hard until the pushrod shatters the piston, then the pedal would have potential for no resistance.

I'm not doubting you mate, but that would take some incredible leg power! It is, after all, a safety critical item and should be built NOT to fail under any normal circumstance and then at least a 100% safety margin.

Steve

[RichardHyde]

Is it possible to bench test a servo before fitting it ?

Do I have to dismantle it to check the Bakelite ?

Are there any serviceable items worth changing while it’s out ?

Thanks, Richard

[xvivalve]

Is it possible to bench test a servo before fitting it ?

The reconditioners can on their test rig, you can't

Do I have to dismantle it to check the Bakelite ?

If it is really badly broken it will rattle when shaken and bits might drop out of the one way valve hole. You can also look through the one way valve hole to look for fractures in the back of the piston, otherwise, yes, it needs to be opened

Are there any serviceable items worth changing while it’s out ?

Yes. Diaphragm rubber, filter, rubber boot and seals etc

Thanks, Richard