Lotus Elan

vexing brake pedal travel problem

PostPost by: LotusEngineeringBuff » Mon Aug 28, 2017 9:08 pm

Hello,
I'm getting close to "finishing" my plus 2. I have driven a few km already and I can now concentrate on the set-up. It turns out the brake pedal travel is to too long. It takes about 6cm of travel until the brakes start to bite.
It does not feel mushy. The pedal just travels freely with no resistance until it starts to feel firm and comfortable.

I checked on the pedal side. The tandem master cylinder looks good (it has been installed new about 10y ago when the previous owner got the car registered in Switzerland from the single M/C UK system). No leak. There is no play in the pushrod. The piston starts to move back as soon as the pedal is activated. The servos look new. They also have been installed new 10y ago and still look bright and shiny. I rotated them the way they should according to what I know.

I have now purged the system twice "by hand" without improvement. I'll try another time tomorrow with a "machine" that put air pressure on top of the M/C tank just to be sure but I doubt there is air in the system because the feel is firm. Just too long a free travel before it happens.

I should say that I have replaced the brake caliper seals and bellows. They have seen no use but I have installed them in 2012.

Thanks for your help
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PostPost by: Melodyk » Mon Aug 28, 2017 9:18 pm

If you exhaust the servos by pressing the brake pedal several times without the engine running do you get a firm brake pedal?
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PostPost by: LotusEngineeringBuff » Mon Aug 28, 2017 9:30 pm

When I press the 1 time, the pedal goes even further. Then, after I pump a few times it stabilizes to about 6cm free travel. I tried both without engine running and with the engine running and I am not sure there is any difference.

tomorrow I'll post a couple of pictures to show how the servos are positioned and the general lay-out. The M/C is sourced from a company called PBR of the like (I cannot rely too much on my memory. I'll put that in writing when I see it next time.
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PostPost by: Melodyk » Mon Aug 28, 2017 9:49 pm

If you exhaust the servos by pumping the brake pedal with engine off the pedal should come up hard. If it doesn't then you quite likely have air in the hydraulic system.
With the servos exhausted you should put your foot on the brake pedal hard and start the engine. The vacuum will build up in the servos and the brake pedal will go down.
It is possible that as the vehicle has a tandem master cyl fitted you have two remote servos fitted. I think this will increase the pedal travel. You could disconnect one of the vac pipes from one servo and plug the pipe and try the same again to see if this makes the pedal better.
If it's air in the hydraulic circuits you could try clamping the flexi hoses to see if it makes any difference. If the air is in the line to the servos this won't help you
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PostPost by: mbell » Mon Aug 28, 2017 9:54 pm

What size(diameter) is the master cylinder?
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PostPost by: LotusEngineeringBuff » Mon Aug 28, 2017 10:22 pm

mbell wrote:What size(diameter) is the master cylinder?


Don't know. but I am happy to tell that the invoice from "the Elan Factory", Australia i have says ITEM No: BRA109
I should call them to ask
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PostPost by: steve.thomas » Tue Aug 29, 2017 5:10 pm

As you have changed the pistons and seals the problem might be that the pistons are retracting too far and giving you an initial long pedal travel to move them out and engage the pads. To rectify this remove one pad at a time and press the brake pedal gently to move the piston out slightly. Then push the piston back only just enough to enable you to refit the pad. Repeat for the other pads and hopefully you have a solid pedal.
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PostPost by: StressCraxx » Tue Aug 29, 2017 6:29 pm

Good advice so far.

Put the car up on stands and remove the wheels. Look at each caliper for pad to rotor and piston to pad clearances. Have someone sit in the car and push the brake pedal upon your instruction. Often, worn pads can cause long travel because the caliper seals and dust boots want to retract. The pads should barely drag on the rotors.

Check your rotors for lateral runout or bearing looseness. Either will cause pad "knockback" and cause a long pedal until you pump them back up again. The bearing clearance spec and rotor runout spec are in the shop manual.

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PostPost by: Foxie » Tue Aug 29, 2017 7:22 pm

StressCraxx wrote:
Check your rotors for lateral runout or bearing looseness. Either will cause pad "knockback" and cause a long pedal until you pump them back up again. The bearing clearance spec and rotor runout spec are in the shop manual.

Dan


Yes, rotor runout/ loose bearings don't help at all. Willwood supply knock-back springs which fit inside the pistons to counter seal pull back. I found them a big improvement. :)
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PostPost by: nmauduit » Wed Aug 30, 2017 10:24 am

LotusEngineeringBuff wrote:It turns out the brake pedal travel is to too long. It takes about 6cm of travel until the brakes start to bite.
It does not feel mushy. The pedal just travels freely with no resistance until it starts to feel firm and comfortable.

I have now purged the system twice "by hand" without improvement. I'll try another time tomorrow with a "machine" that put air pressure on top of the M/C tank just to be sure but I doubt there is air in the system because the feel is firm. Just too long a free travel before it happens.

I should say that I have replaced the brake caliper seals and bellows. They have seen no use but I have installed them in 2012.


Does the pedal feeling change (improving) when you rapidly pump twice the pedal ? if so I would go for air in the system, suspecting small bubbles trapped into the top of the Girling 16 front calipers (this can get worse with silicone DOT 5 brake fluid, which seems prone to very small bubbles). If a rapid "machine" purge does not manage to push the air away, I would try to undo the calipers from the car to purge them at an angle (may be easier with an helper).

I've also had tandem MC issues in the past, intermittent long pedal push ending with partial braking : a filament of rubber had been cut out from the piston seal and was occasionally keeping the fluid return valve at the end of the MC slightly opened.

disclaimer : none of the above was servo related.
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PostPost by: LotusEngineeringBuff » Wed Aug 30, 2017 1:32 pm

oil_purge_press.jpg and
Small custom clamp and cover to use a compressed air brake purging "machine"
nmauduit wrote:
Does the pedal feeling change (improving) when you rapidly pump twice the pedal ? if so I would go for air in the system, suspecting small bubbles trapped into the top of the Girling 16 front calipers (this can get worse with silicone DOT 5 brake fluid, which seems prone to very small bubbles). If a rapid "machine" purge does not manage to push the air away, I would try to undo the calipers from the car to purge them at an angle (may be easier with an helper).

I've also had tandem MC issues in the past, intermittent long pedal push ending with partial braking : a filament of rubber had been cut out from the piston seal and was occasionally keeping the fluid return valve at the end of the MC slightly opened.

disclaimer : none of the above was servo related.


Good advice from everybody.
I will try to remove any play by removing the pads one at a time and have the pison come out. I would hate to have to take the callipers out and fit the anti knock-back springs (I'll keep that for the end if nothing else works...)

Yes indeed, the travel is a bit shorter the 2nd time the pedal is activated. I will machine purge for good asap (I do not have the proper cover, and I have just made a clamp and a cover to hold the pressure, see picture) and hold the front calliper "at an angle" .

Along the way I did have to properly understand the way a tandem master cylinder works.
https://www.youtube.com/watch?v=2OU150Yskhs

But now I think the behavior to expect when purging a dual tandem system is the following

a) suppose the front circuit has air bubbles or a failure. if I purge the rear system, the pedal will travel freely until the piston acting on the front circuit bottoms down (spring fully compressed). Then the pedal will create pressure in the rear circuit which can be used to expel fluid+bubbles in that circuit.
b) the same behaviour is expected if the rear circuit has a failure and you are purging the front one.
c) if front and rear circuit are fully functional, then there is NO free travel. if the pedal is actived pressure will build up in both circuit and if any purge screw is opened the pedal will only travel the 1st half of the stroke, expelling fluid from that circuit, before pressure builds on the other circuit and the pedal stops.

Right?

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PostPost by: denicholls2 » Wed Aug 30, 2017 3:42 pm

LotusEngineeringBuff wrote:Yes indeed, the travel is a bit shorter the 2nd time the pedal is activated.


Caution: This is normal as long as the difference is not excessive. The resting position of the pads is not against the disc but a bit away from it (so you don't cause a fire due to overheating brakes). Two pumps in succession do not allow the pads to return to their normal resting position, the system is not that responsive because weak rubber gaiters are usually all that is pushing the piston back into the bore when the wheels aren't turning.

Air usually manifests itself as squishiness in addition to pedal travel. On the second pump, the pedal would move less to the point of engagement (per above) but still feel mushy after engagement. This is due to the difference in compressibility between air and brake fluid.

Pumping the pedal in your garage, disc runout shouldn't be a factor, it will manifest itself only when the wheels are turning. So you're left with mechanical play between the pedal and the master cylinder attachment and knockback per the gaiter design on the piston (plus return springs if any are fitted). Or travel resulting from cylinder bore size.
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PostPost by: Craven » Wed Aug 30, 2017 4:21 pm

Long pedal travel is indicative of one of the two brake circuits not functioning correctly.
If sufficient pressure can?t be achieved in one circuit the master cylinder defaults to a mechanical rather than hydraulic coupling to operate the other circuit, this mechanical link only happens after a long pedal travel. It?s quite possible that the rear brakes are not operating correctly without the driver realizing.
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PostPost by: nmauduit » Wed Aug 30, 2017 4:49 pm

denicholls2 wrote:Two pumps in succession do not allow the pads to return to their normal resting position, the system is not that responsive because weak rubber gaiters are usually all that is pushing the piston back into the bore when the wheels aren't turning.


That (when one has a stiff gaiters issue), and I'm suspecting the MC return spring being stiffer than the trapped air bubbles, which enables to pull a bit spare fluid from the tank(s) after the first stroke and to push that extra fluid during the second stroke : that would reduce the trapped air effective volume, thus improving the pedal feel...

regarding the video, there is a little difference in the Girling MC used on elans and what is displayed, in that the front tank input and front pressure circuit locations are switched (it is a spring operated valve inside the MC at its end that seals the front circuit, that is the circuit away from the firewall).
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PostPost by: RichardHawkins » Fri Sep 01, 2017 7:11 am

Francois,

I am nervous to add my comment as I am not particularly knowledgeable on this subject. Some vehicles that I have experienced with a soft long travel pedal, did not respond to conventional gentle depression of the brake pedal. The solution was simple. All I needed to do was depress the pedal quickly to purge air from the system. This sounds brutal, but stamp on the pedal like your life depends on it. One of my Lotus friends had problems with his Plus 2 that sounds similar to your situation. Stamping on the pedal solved his problem.

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