14 replies to this topic

[cal844]

Hiya folks

My 93 sprite has a rock solid brake pedal, fantastic brakes but only about 3/8th an inch 'play' before the brakes are working.

When I do the test to check the servo ( pump pedal pressure up, then hold it on and start engine, pedal should drop an inch or 2) nothing moved...


Tiny amount of fluid in the servo can, vacuum pipe is clear (removed pipe from. Servo, plugged with my finger then removed and the car misfires as expected due to air leak)


Have changed the servo (green tag 88 on type). 2nd hand, has been in a box for 5 years, was tested and working before storing.

Master cylinder is a couple of years old.

I think the pipe has now been changed too.

Can it be the valve on the bulkhead?!


Anything else to test before I fire the new parts cannon?

Thanks

Cal

[weef]

The problem could be mechanical, pedal pivot or master cylinder, or relating to the servo assistance.

First off I would crack open a bleed screw, start the engine and press the pedal, it should go to the floor and fluid forced out of the bleed screw.

Dependant on the result dictates how to proceed. If it does not go to the floor look at the pedal pivot,clevis pin or anything else that might inhibit the action. You could unbolt the master cylinder from the servo body to check the throw of the pedal, it should go to the floor, and you can check the master cylinder action, again, by opening a bleed screw and using something like a screwdriver push the cylinder piston in, it should move smoothly in and fluid will be forced of the bleed screw.

If nothing is found using the above suspect the vacuum assistance. To check this you will need some test equipment, a vacuum gauge to monitor the vacuum and a pump capable of generating a vacuum.

Plumb in the gauge on the servo side of the pipe one way valve that feeds the servo vacuum chamber and start the engine. Once the vacuum is steady stop the engine. The vacuum should hold only leaking off when the brake pedal is pressed. If this is not the case you will have to split the pipe and fit the vacuum pump to the pipe that feeds the servo body, servo side of the one way valve, leaving the gauge in circuit to monitor the vacuum. Pull a vacuum using the pump and monitor, it should hold steady, if not suspect the servo diaphragm or the air admitance valve. 

If all is OK using these checks suspect the the pump circuit/ one way valve and any other vacuum fed components fitted, again checking integrity using the vacuum gauge.

Once finished ensure you reinstate the braking circuit to its servicable condition.

[cal844]

Thanks for the response!

I'll check the servo assistance during the week, the brakes work great (can stall the car, when pulling away with the footbrake pressed) just the servo isn't working.

Edited

We had the drums off the rear for replacement as the OE factory drums were ovalled, checked rear cylinders and all is well.

Edited by cal844, 20 May 2023 - 07:25 PM.

[cal844]

We have replaced the old OE servo with a brand new unit from Bull Motif Spares, still the same BUT brakes are awesome.

Any ideas on what to try next?

[weef]

You will have to measure what vacuum is exerted on the servo chamber and go from there.

[cal844]

You will have to measure what vacuum is exerted on the servo chamber and go from there.

Even on a brand new part? If there isn't enough vacuum being generated, what do we look at?

[weef]

If it is a standard carburetor/inlet manifold the vacuum depression is produced by the inlet manifold. Follow the pipe that comes from the servo chamber back. Check this pipe is clear of obstructions/ cracks, a one way valve is fitted/ working and fitted so that it will draw air from the servo chamber and check the tapping on the manifold is tight and free of instruction.

[cal844]

If it is a standard carburetor/inlet manifold the vacuum depression is produced by the inlet manifold. Follow the pipe that comes from the servo chamber back. Check this pipe is clear of obstructions/ cracks, a one way valve is fitted/ working and fitted so that it will draw air from the servo chamber and check the tapping on the manifold is tight and free of instruction.

Yes it's a HIF38 carb car (with charcoal canister delete)
Vacuum pipe and one way valve is working fine.

I disconnected the hose from the servo, plugged the end with my thumb and started the engine, then released my thumb and the engine misfired massively (can stop the misfire by plugging the hose again), we even refaced the 2 copper washers to shiny metal.

Can the one way valve be partially blocked?

[sonscar]

Is it possible that there is insufficient pedal movement to actuate the servo?Does it stop with less effort when the servo is connected Vs disconnected?Just thoughts,Steve..

[viz139]

The push in pipe connection on the manifold can leak. I found applying some grease to the end of the plastic pipe can help it seal.

[Lplus]

We have replaced the old OE servo with a brand new unit from Bull Motif Spares, still the same BUT brakes are awesome.

Any ideas on what to try next?

If the brakes are really that awesome I'd wonder if the servo is actually working but doesn't respond to your test in a manner you expect.

[weef]

You will have to measure the vacuum at the servo.

Tee piece a vacuum gauge in the pipe between the one way valve and the servo body.

Start the engine and let the vacuum develop.

Stop the engine and the vacuum should hold, indefinately.

Press and release the brake pedal and the vacuum should drop then hold. It usually takes two or three quick presses to empty the servo chamber.

This is how the system operates.

Try this and see if it responds as explained.

[weef]

Just had a thought, you could remove one brake pad and fit a piece of wood maybe half thickness of the removed pad. Now pressing the brake pedal should give a fair bit of travel as the caliper takes up this new "pad" thickness, and you can see what happens. Once the caliper pot reaches the piece of wood the pedal should again be near the top. This operation might help you monitor the servo vacuuum and see what is wrong.

[cal844]

Just had a thought, you could remove one brake pad and fit a piece of wood maybe half thickness of the removed pad. Now pressing the brake pedal should give a fair bit of travel as the caliper takes up this new "pad" thickness, and you can see what happens. Once the caliper pot reaches the piece of wood the pedal should again be near the top. This operation might help you monitor the servo vacuuum and see what is wrong.

Good idea! I'll try that at the weekend, MOT tomorrow 😂

[andyapanel]

I had the grommet fail an hour before its MOT. Easy fix if you have the grommet, annoying if you don't.

I reckon the actual servo is pretty bullet proof.

If you are used to a servo, the brakes feel very different, even if they are working fine.

Good luck