Brake servo (booster) problem

Just got my car back on the road with an almosy completly new braking system and it's awful (see my other post in all GT40)I took the car for a ten mile run to get the brakes bedding in, but the brakes are like stamping on a block of wood when you first hit them, but if I take my foot off and hit them again they are a bit softer and brake better. I'm confused.
I lifted the front clip with the engine running and after turning the engine off, could hear air hissing from one of the brake servos for a few seconds.
I rebuilt the servos with service kits, have I done something wrong? How can I test them?

Simon,
Can the brake pedal travel full length without presure or while bleeding, i once re-assembled a m/c after rebuilding it, there was a retaining screw for the piston that i put in before the piston (doh) and it stopped the pedal travelling all the way in. All i had to do was undo the retaing screw then fully press the brake tighten up screw and all was sweet. I'm not sure of the set up youre running but i hope it helps, pulling under braking can also be due to worn wheel bearings.

Steve

Steve,

the master cylinders are both new (wilwood) and untouched. I bled the system (several times) with an ezi-bleed to remove all the air.
I'm quite confident that it's clear of air but it seems to me as if one of the servo's is not working?
How do I test them?
Simon

It sounds like your vacuum is inadequate. Make sure that your lines are not collapsing. If you are not using a vacuum reservoir mounted near the booster, it's time to think about it.

TO test them, normally you just start the car, then shut if off, wait 30 seconds or so then hit the brakes, you should get at least 1 full application with assist and probably a partial second one till the pedal becomes real hard. Now I am going to guess that yours are remote from the pedal, I've never had to check one of them but this should work, run the car, shut it off, wait 30 seconds and pull the vacuum line off, if there is vacuum, then they arent leaking.

Looks like I've got a leaking servo.
Pulled the hoses off to test vacuum with the engine running, Lots of vacuum in the lines (new hoses)
Put a spare hose on the rear servo and sucked hard (doncha just love the taste of rubber pipe...yuk)
Rear servo, no problem, lots of vacuum, front servo....leak !
Damn have to take the whole thing apart. Any ideas what I could have done wrong when I rebuilt itThanks

Simon

I have only ever had a Honda one apart. On that one, the only thing that could cause a leak is the bellows not properly in place or torn.

I emptied both brake circuits (F&R) took off the rear servo and discovered that there was a slight leak around the vacuum pipe that connects the bellows to the front of the servo.
I put a new jubilee clip around it and the problem was solved.
I then put it all back together and bled all the air out of the brakes but I'm still not happy with the brakes.
The first application with the engine running, the pedal moves down as it should (like my normal road car) the second prod, immediatly after the first the pedal is really hard as if all the vacuum has gone out of the system. If I leave it a second or two (still with the engine running) it seems here is more vacuum built up and it feels fine again. Do I REALLY need a vacuum reservoir? the servos are the same as I had on the car before and I didn't notice this problem then with the standard (twin circuit) GTD system.
With the engine off and the vacuum gone, when I push the pedal, there's also a small "clonking" noise from just one of the servos, allong with a very slight feeling in the pedal (don't know how to describe it)
With my braking related "mishap" still fresh in my mind, I not going to drive the car agin until this is well and truly sorted.
To this end, I'm going to strip down the braking system again and pull the servo's apart. Is there a way of testing them (not just the vacuum) or can anyone recommend a brake expert in my area (SE UK)
Thanks, any advice greatly appreciated.

Simon

Simon,
I reckon you still have a leak somewhere, which is preventing the servo recovering in time for the second brake application.
IMHO, you don't need a vacuum reservoir - solve the original problem first.
My servo difficulties were sorted quickly and cheaply by Frank Catt. Give him a shout, he's seen most of them before!

Just been down to the garage to try once more before I dismantle the whole lot.
This is what I noted.
Both servo's hold a vacuum after the engine is turned off. after a few stokes of the brake pedal, the vacuum on the Front brakes looses vacuum (as it should I suppose) while the rear brake servo still has (what seems like full) vacuum.
This leads me to think that the rear servo is not doing it's job properly even though the vacuum side of it is working fine.
I guess it's possible that I have rebuilt the servo incorrectly.
Simon

Tony, I think this http://www.gt40s.com/ubbthreads/showflat...=true#Post38642 thread may be significant !!!!!!

Simon

I'm pretty sure I've sussed it.
I rebuilt the servos using all new parts and changed the fluid from silicon (dot 5) to non silicon (dot 5.1) but didn't realise I'd missed out on replacing the two small rubbers on the non return valve in the servo.
As everything else was new/reconditioned there's not a problem with the system as a whole, but these seals have swollen and are causing the valves to stick in the bores.
New seals on the way, so should be sorted soon.

Simon

Here's a (pretty poor quality) picture of the offending part.
You can just about make out that the top seal is slightly bigger than it should be.
Simon

Simon,

If the change of fluid type has caused the problem (?) then are other seals in the system also at risk - most notably the piston seals on the calipers - or are these all new.

I know you need to empty and ideally flush out any system when you change the type of oil (e.g. mineral to synthetic) but I didn't realise it would have this effect.

Dave,

Evey other seal in the system is new and has only been exposed to dot 5.1 fluid.
I spoke to a specialist today, a chap called paul norton. What he doesn't know about servo's isn't worth knowing.
He told me that this can occasionally happen as the two types of fluid have different "swelling" properties. He also said it's very rare and that he has been using both fluids in his cars for some time with no ill effects.
Got some new seals on the way.

Simon

Simon
I know this is an old post, but I was hoping to pick your brains....

Neither of my servo's were working, giving the typical "dead pedal" feeling. Having checked the vacuum, and having read up on this forum (and others), I think that I've had the same problem as you with the sticking control piston (the small one with 2x O-rings). Both pistons were solid when I opened them up, probably caused by long periods of inactivity.

I've cleaned everything and replaced all of the gaskets and rubbers. My query is: how stiff should the small piston be? I know it has to be a tight fit, but mine seem to be reasonably stiff, and I can't see what makes them return to their "closed" position. If they are stuck open I'll never get a sealed vacuum, if they are stuck closed, I'll never get servo assistance.

Any thoughts would be appreciated

Darren

Darren, the bore into which the small piston fits frequently gets lightly rusted, so use a pencil or similar wrapped with 1200 grit wet and dry, use WD40 to lubricate and hone out the bore until you have a nice sliding fit with the piston. Should do the trick. Frank

Frank

Thanks for your reply. The cylinder and piston were in prety good condition, bar a tiny bit of rust at the very top of the cylinder. As the O rings don't travel as far along as the rust, I think I should be OK.

A chap in our metal fabrication shop at work kindly polished the cylinders for me - they looked new once he'd finished. However, I was just suprised that, on fitting the new O rings, the piston was still quite stiff (and yes, I did remember to coat the O rings with brake fluid before inserting them).

I did read on a MG forum that some chaps were removing one of the O rings to prevent the piston from binding so easily, but I'm not brave enough for that!

I basically just wanted some reassurance that the piston should be a tight fit, before I re-insall the whole system.

Darren

You can not mix silicon brake fluid with standard brake fluid in ANY amount in ANY part of the brake system EVER. Not a DROP..... NONE!!!

If I had to convert a car from silicon to normal fluid I would disassemble EVERYTHING down to the component parts and replace everything that isn't metal. ALL seals, o-rings ALL of them. Then I would pump brake-clean through the piping until I was SURE that all the brake fluid was washed out. Then I would blow compressed air through the piping to dry it from the brake-clean.

Converting silicon brake systems nearly always results in bad brakes if parts are reused without complete cleaning and rebuild. Until you have done this you will just keep ruining new parts.

I will never use silicon brake fluid just for this reason alone.There's no real reason to use it except in full on race cars.

Howard
Sorry - perhaps I wasn't clear in my post. I havent switched from standard brake fluid, but I have re-built the boosters as did Simon, having suffered the same "dead pedal" as he commented on. I was wondering how stiff the control piston should be as mine seem to be reasonably solid despite fitting new O rings. Panic over!

Darren