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[Robin]

Really   

All the servo does is reduce the foot pressure required to operate the brakes.
Without vacuum the brakes still work, just require more foot pressure. That's how they are designed, as a fail-safe.

Now if you're talking about hydropneumatic brakes, such as the old Citroen Xantia, they didn't work at all without hydraulic pressure - the foot brake was effectively a valve which allowed stored hydraulic pressure to operate the brakes - no pressure, no brakes no matter how hard you stamp on the pedal   
It's one of the reasons why the handbrake on the Xantia operated the front brakes, not rear.

[Wittsend]

Robin is quite correct ...

 

Important safety message here:

Your brakes will still work and stop you in all situations if the vacuum fails on the servo.

 

[w3526602]

Now if you're talking about hydropneumatic brakes, such as the old Citroen Xantia, they didn't work at all without hydraulic pressure - the foot brake was effectively a valve which allowed stored hydraulic pressure to operate the brakes

Hi Robin,

How true! I had a Citroen BX with the hydraulic suspension and brakes. Start the engine and the car would go up, although you could control how high. It suffered from dud "balls", and leaky rubber pipes. Luckily, the steering was not power assisted. I replaced the balls (£££) but was scared off by the complicated pipes enema hoses that connected everything together. I think it drank about a litre of the special fluid per week ... at something like £15 a litre. I was very glad when somebody crashed into the back of me.

Yes, no brake pedal, just a rubber button thingy on the floor. I can't remember which failed first ... hydraulic suspension, which would drop you down to about 4" ground clearance (avoid speed bumps), or the brakes, which just stopped working. Character building. 

Occasionally I ponder on the little V-belt driven pump, that sat on top of the engine. Can my memory be correct ... a  pressure in four figures? Hmmm! Landy FFR engine with double crank pulley, and mount the pump where the 24V alternator sits.

But what would you do with all that pressure?
Just me, you understand, but I don't like the TRE in the middle of the track-rod.

Should I ever decide to have another S2 (I'm still yearning) and go for PAS, I would look into fitting a LHD drag-arm to the left hand swivel (so there are two drag-arms) and connect the hydraulic ram to the outer "eye" on the second  drag-arm.

I have considered (but not in any depth) using the above idea, but with a  power assisted rack and pinion bolted to the front bumper.  Hey, that would work, in theory, using the original TRE "eyes". It might be a good idea to weld the bumper to the dumb-irons, and it would need a steering column with a couple of U/Js to avoid impalement.

To avoid bump-steer, it might be better to connect the rack to just one drag-arm, and retain the original track-rod.

Probably time to get my coat.

602

[Herald1360]

Really   

All the servo does is reduce the foot pressure required to operate the brakes.
Without vacuum the brakes still work, just require more foot pressure. That's how they are designed, as a fail-safe.

True enough, and in the case of a Land Rover, if the M/C diameter is the same as for the unservoed version, the brakes with no servo action should work as for a normal non-servo setup. I suspect though, that for cars with a servo as standard and disc brakes, advantage has been taken of the servo to increase the M/C bore to reduce pedal travel resulting in the apparent loss of brakes if the servo fails resulting from the very large step change increase in pedal pressure required.

[GlenAnderson]

There is a little more to it than just reducing pedal pressure although, granted, with remote servos that’s all you get.

The direct servo has a longer lever, and a different ratio between pedal, cylinder and fulcrum. This has the effect of also lowering pedal pressure. The return springs are also lighter, which reduces effort too. You also have the benefit of a direct servo providing its assistance from the instant you apply the brakes, which means they’re more linear in operation than remote ones which require a certain amount of line pressure to start to assist. Finally, in the case of fitment of a dual circuit master cylinder, you have an element of front/rear proportioning which can aid stability (as well as the security of a fail safe).

The pedal mounted servo and dual circuit master cylinder setup on my 109” provides significantly more feel, with much less pedal effort, than the standard single circuit system it replaced.

There is a line to be walked between originality, cost, performance, mechanical ability and vehicle usage. Only you can decide how to navigate it. 

[Wittsend]

Just remember with these home brew brake modifications to inform your insurance company and be prepared for an engineer's inspection.
Fitting an "off-the-self" power steering kit should be OK as someone has done the engineering calculations and tests - but your insurers need to know.

You can't give them any chance to wriggle out of settling your claim.

[w3526602]

Your brakes will still work and stop you in all situations if the vacuum fails on the servo.

Hi Alan,

Have you tried.

Moskvich brakes certainly did NOT work if you didn't have vacuum. And TOO powerful if the engine was running. You could recognise a new Moskvich owner by the way they approached traffic lights in a series of short skids, (I'm not joking).

Barbara's incident with the cliff happened when she just dropped the hand-brake on our Triumph 2500TC, and let it roll. When she met the lorry coming up the hill, she hit the brake pedal. Nothing happened ... no brakes. The folowing day I flagged down a passing milk lorry, got him to pull me off the tree, and up the cliff. She started easily, and I drove it home, despite the passenger side of the roof touching the passenger door skin.

I think the E-type was introduced in 1971. A dentist a few miles South of Croydon bought one. Late one night, his three sons (eldest 12) were killed when they took it for a drive (or whatever ) I'm assuming they let it roll down the long hill, with a dead engine ... and found they had no brakes. Shoving it into gear and lfting the clutch would have turned the engine ... even if it didn't start ... and given them brakes.

We bought a Daimler 2.5 V8 (same shape as Inspector Morse's Jaguar). The brakes only worked, sort of if the engine was running, despite it having a vacuum reservoir. I found a hole in the reservoir, so I by-passed it by joining the in and out pipes together., after which the brakes were akin to a "mighty hand" grabbing the back bumper ... but they still didn't work if the engine wasn't running. And the hand brake never worked, par for the course on that model, even when new, (and acknowledged by MOT testers).

I will agree that if you have a reservoir, you will have enough vacuum for four or five good stops. And I'll admit that I haven't done my "test" on a modern car. Remind me tomorrow.

602.

[Peter Holden]

Even the smallest modern car seems to require assistance for brakes and steering.  We have a Berlingo based camper (and a Berlingo car).  Recently the power steering pump decided to malfunction on a journey, gosh the steering was heavy, think a loaded land rover with flat tyres and the brakes are non existent however much you stand on the pedal if the engine is not running but the vac pump is tiny.  The brakes on tha car which is a later model are the same and trying to turn the steering without engine power is a no no.

On a SWB land rover I find 11" TLS brakes on the front, standard 10" brakes on the rear and a 1.9. to 1 remote servo an excellent combination and yes I did tell the insurance company whose reply was that is a standard upgrade, no problem.

Peter

[w3526602]

You can't give them any chance to wriggle out of settling your claim.

Hi Alan,

Insurers find it very difficult to wriggle out of Third Party  claims.

Provided that you are not doing anything that is prohibited (detailed on your CERTIFICATE of Insurance, you are covered. Those exemptions include ...

1. Driving without a licence (or never having held a licence) to drive that particular CLASS of vehicle.

1.1. I would argue that towing a trailer that takes your train weight over your licence entitlement  will mean you are driving without a licence, and therefore not insured. But hey, what do I know? I think it will give them the opportunity to "void" any "own damage" cover.

1.2. I would also "look sideways" at a Cat.B. licence holder driving a 12-seat Land Rover Safari. Your Insurers may get a little "po-faced" if you have more than eight seats. Will the passenger liability factor affect your insurance premium? Dunno! I have a vague memory, from many years ago, about discussing this with my insurers. I think I agreed to remove the inward-facing rear seats from my Safari, which left me with 5/6 front facing seats. Not a problem ... my two GSDs didn't use those seats, anyway.

2. I think insurers do not like modifications that "enhance performance". Go look  at your Certificate of Insurance (both sides) ... which is the only document that counts ... the policy falls into the "bumf" category. But read it (once) so you know what's covered, NCD, etc.

602

Overtaken by Peter ...

Barbara's 2.5TC engined Stag blew it's home brewed high pressure hose on the power steering ... front wheel hit a pot-hole when at full lock, albeit at a walking speed. Impressive steam/smoke when the PAS fluid hit the exhaust.

I had to drive it home, the steering was too heavy for her.

My Citroen BX was scary to drive, due to a hydraulic leak (I think at £15 per litre). I didn't even have a pedal to pump and swear at, and ground clearance measured in "thickness of fag packets". Characterb building! Luckily, I didn't have the PAS model.

The BX had a lovely little hydraulic pump, almost fit in your pocket. I reckon it would not be difficult to mount onto a Series engine, using a double-V crank pulley. But what could you do with all that lovely high pressure ( I think it was over 1000lb/in2). The suspension spheres only had a short travel ... but how short?

A car suspended on "enema" hoses. The mind boggles.

602

[w3526602]

Hi,l

Completely off the wall, not researched, and not thought through.

What happens if you swap the top and bottom arms on the relay? I think they are different lengths.

I suspect that radial tyres will make the steering heavier. And so will bigger tyres?

Altering the castor angle by fitting spring wedges will alter make the steering heavier (or lighter if you are not careful)

Tired rear springs will alter the castor angle. Longer rear swinging shackles will lighten the steering, but maybe not enough to be noticable.

Are you sure your steering is heavy not stiff

Tyre pressures? What IS the correct TPI? I've had tyre bays insisting they should be 4O+TPI I run at 25TPI on both 205 and 235.

Do a TESCO car park self-centering test, both locks.

602

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