Dragging brakes.

Talk about the E-Type Series 2

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Tom W
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#1 Dragging brakes.

Post by Tom W » Fri Jul 26, 2019 4:56 pm

Has anyone experienced sticking front calipers on the S2, particularly with high ambient temperature? I went for a drive yesterday in the 35degree heat. I wasn’t driving quickly, very slowly if anything, as I was with a group of several other classics, so not more than about 40 mph, and certainly not hard braking. The engine was running warm, but not over heating, though under bonnet temperatures were higher than normal.

After driving for a while, the front brakes started dragging. Very soon afterwards, I had to pull over, with the front brakes very hot and dragging heavily. Now my first thought was to the usual problems of either the reaction valve or servo. However, I’m not sure this was the case for the following reasons:

I removed the cap from the reaction valve. The valve was sealing properly, in the off position. So no inadvertent triggering of the servo.

With the onset of the dragging brakes, the brake pedal had developed a growing amount of free play at the beginning of its travel. This would suggest possibly the shuttle valve in the servo was sticking at the end of its travel, and not being returned by the internal spring However, the 2nd circuit on my car feeds the rear brakes. As it were the front brakes that dragged, this couldn’t be the case.

I don’t believe it’s a problem with the master cylinder sticking and failing to return. This would support the free play experience at the pedal, but if there were enough pressure in the master cylinder to lock on the brakes, then the reaction valve would also be actuated. Also, the rear brakes would be dragging too.

I don’t think it’s a lack of free play at the master cylinder push rod, preventing the master cylinder from venting to the reservoir. If this were the case, the pedal would feel normal, not with the growing free travel, and the reaction valve would be operating.

Which leads me to think it’s either something physically sticking in the front calipers, the front brake lines collapsing internally and trapping fluid, or something in the hard line acting as a one way valve between the servo and front calipers.

What’s curious is why it affects both calipers simultaneously, and evenly, and also, why ambient temperatures caused the problem rather than heat generated from heavy braking. Cracking off a front bleed nipple allowed the pressure to release, in both calipers. That points to the problem being hydraulic, and somewhere in the hardline between the servo and the T-piece.

My car has a tubular manifold. I will check that the brake line isn’t running so close to the manifold as to be heat affected. Though, if the fluid were to boil, surely I’d have no brakes, not locked on brakes. I run DOT5.1, which has a wet boiling point of 180deg C, so the brake line would have to be extremely hot to boil the fluid.

All of which brings me to think there must be some detritus in the hard line, which expands and wedges with heat. I’ve ordered some new Goodridge flexi hoses for the fronts as a precaution. I will replace the front hard lines at the same time, flush the servo, and clean out the front T-piece.

Anyone else got any thoughts?
Tom
1970 S2 FHC

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politeperson
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#2 Re: Dragging brakes.

Post by politeperson » Fri Jul 26, 2019 5:12 pm

How old is your brake fluid? Maybe worth changing it, as it absorbs water and expands when hot. Old brake fluid expands when hot though water absorption. Water turning to steam.

All exacerbated by a sticking reaction valve piston and caliper pistons that are (relatively) infrequently used.

Maybe knackered flexi hoses as you say, restricting the return flow.

On the assumption your master/rod clearance is OK, I would change the hoses, change the fluid and check and clean the reaction valve piston again.

All this costs peanuts.

If the brakes are still grabbing when hot, it is probably a sticking caliper.

To check them - touch them individually after a drive. you will soon find the hot one!

If you are lucky, exercising the pistons with no pads in can make them loosen up, however I would send the offending items off to Past Parts who send them back again gleaming and reconditioned for £70 odd quid a caliper.

However removing the rear brakes is a PIA. Better exercise than joining a gym though.
James

L.J.K. Setright was right.
"You just cant beat a good E-type"

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#3 Re: Dragging brakes.

Post by Tom W » Fri Jul 26, 2019 5:55 pm

The brake fluid is only 12 months old. All replaced when I rebuilt the IRS last year. Similarly, rear calipers were rebuilt when that job was done.

The reaction valve piston was removed and cleaned a couple of years ago, but I don’t think that’s the cause as the reaction valve was seated when I took the cover off, so not activating the servo.

I’m also dubious of it being a sticking caliper as both brakes got equally hot and dragged evenly.
Tom
1970 S2 FHC

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Nigel GT
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#4 Re: Dragging brakes.

Post by Nigel GT » Fri Jul 26, 2019 7:14 pm

Hi Tom,
Have you checked the servo vacuum hoses that run past the exhaust, I had some collapse in high temperatures, just run your hands along the hoses and you can feel it.

Nigel

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#5 Re: Dragging brakes.

Post by Tom W » Fri Jul 26, 2019 7:26 pm

That’s a possibility, but the servo hoses are recently replaced, and with proper vacuum hose. They’re also insulated with aluminium corrugated insulation hose.
Tom
1970 S2 FHC

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johnetype
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#6 Re: Dragging brakes.

Post by johnetype » Fri Jul 26, 2019 8:02 pm

I would vote for internals in the remote servo sticking. We already know the servo is susceptible to problems when hot hence the need for an exhaust manifold heat shield.

Which circuit in the brake servo are your front brakes connected to?
John

1969 Series 2 FHC

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#7 Re: Dragging brakes.

Post by Tom W » Fri Jul 26, 2019 8:38 pm

The front brakes are fed from the primary circuit on the master cylinder.
Tom
1970 S2 FHC

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johnetype
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#8 Re: Dragging brakes.

Post by johnetype » Fri Jul 26, 2019 9:29 pm

Tom,

There is only one circuit on the master cylinder, the system doesn't split into front and rear until the remote servo unit. If the front brakes are connected to the rear of the remote servo then yes, that's the primary circuit and I can't see how that could cause your problem, but if the rear brakes were connected to the rear circuit on the brake servo then a sticking valve within the brake servo could leave the brakes connected to the front part of the brake servo sticking on.
John

1969 Series 2 FHC

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#9 Re: Dragging brakes.

Post by Tom W » Fri Jul 26, 2019 9:44 pm

Sorry, my typo. Meant to write the front brakes are fed from the primary circuit on the servo, the one nearest the vacuum diaphragm, i.e. the same circuit as the master cylinder.
Tom
1970 S2 FHC

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abowie
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#10 Re: Dragging brakes.

Post by abowie » Fri Jul 26, 2019 10:39 pm

Tom W wrote:
Fri Jul 26, 2019 5:55 pm

The reaction valve piston was removed and cleaned a couple of years ago,
Still worth taking it out and generously lubing it IMO.
Andrew.
881824, 1E21538. 889457..oops
http://www.projectetype.com/index.php/the-blog.html
Adelaide, Australia

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#11 Re: Dragging brakes.

Post by ralphr1780 » Sat Jul 27, 2019 5:24 am

Tom, how old are the brake pads and which brand?
I had exactly the same behaviour few years back: the pads were a tight fit in the calipers and the iron had rusted a bit. With increasing temp they were binding progressively and would only release when cooled down. Dismantling, cleaning and filing the metal base cured the issue until new quality pads were fitted.
Ralph
'69 OTS + '62 OTS - Belgium

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#12 Re: Dragging brakes.

Post by Tom W » Sat Jul 27, 2019 7:56 am

The front pads were changed 4 years ago, the rears were new last year when I did the IRS. I forget the make of the fronts, the rears are what SNGB sell as OEM. I will clean up the front calipers and check everything is free. I don’t believe it’s mechanical binding though. Cracking off a bleed nipple freed things off. Also, very steady driving in high ambient temperatures causes the problem, but heavy braking on a normal day, to put heat directly into the calipers, doesn’t.

Researching further on this, there is another potential cause that would fit with the symptoms I experienced. That is that the slave piston (not the shuttle) in the servo is failing to return after the vacuum is removed. This would mean the rears were sticking on too. It was obvious the fronts were dragging, but I didn’t actually get underneath and check the temperature of the rears. If the slave piston were sticking, this would explain the free play I experienced at the pedal.
Tom
1970 S2 FHC

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42south
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#13 Re: Dragging brakes.

Post by 42south » Wed Nov 06, 2019 7:28 am

hi guys
Tom how did you get on with your diagnosis of the brake problem. I have almost identical symptoms, with the front brakes sticking I suspected the booster so replaced it, but the symptoms persist.
Im going to lubricate the master cylinder reaction valve next. Thinking of dismantling and cleaning front calipers then fitting new pads.
Any other suggestions
cheers
mark
Mark Brown
1971 S3 Etype
Son: when I grow up, I want to be a pilot
Dad: You’ll have to choose.

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#14 Re: Dragging brakes.

Post by Tom W » Wed Nov 06, 2019 9:16 pm

I completely stripped the master cylinder. This turned out to have some pitting on the bore of the reaction valve cylinder so I replaced it with a brand new item. I also rebuilt the servo, which when stripped had a quantity of brake fluid in the vacuum chamber. I don’t know if this came from the pushrod seal, or from the reaction valve piston on the master and leaked down.

At the same time, I took the opportunity to replace the remaining original brake lines, and also the front flexi hoses, though I didn’t find any faults with these that would have caused the binding.

I suspect my fault was caused by the pitting on the master cylinder reaction valve bore. This is what I think was happening. The reaction valve piston doesn’t move very far at all, and ultimately it’s return to rest position is only provided by the vacuum on the reaction valve diaphragm. The spring in the reaction valve only closes the forward valve chamber to atmosphere. At this point, there will still be a pressure differential between the front and rear chambers of the reaction valve. This pressure differential causes the reaction valve diaphragm (and piston) to move rearwards, until the valve in the diaphragm opens and the pressure equalises. Once the pressure equalises, the pressure in the servo vacuum equalises too, and no force is applied to the hydraulic circuit. However, if the piston sticks, the pressure across the reaction valve never
Fully equalises, and so the servo is always slightly applied. This will apply all 4 brakes, but maybe the rears being onboard and smaller are less noticeable.

The only thing I don’t yet have an explanation for is why the pedal travel increased as the brakes began dragging. The reservoir should fill any space caused by the servo piston moving on its own.

Since I rebuilt everything we’ve not had the same hot weather that caused my problem, but the car did perform without fault on the Goodwood revival trip. So I can’t say for sure that I’ve resolved it, but so far it looks promising.
Tom
1970 S2 FHC

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42south
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#15 Re: Dragging brakes.

Post by 42south » Fri Nov 08, 2019 5:47 am

Hi Tom
Thanks for your detailed analysis.
My front brakes were sticking after a hot drive, so I thought it must be the servo with a sticking piston, so replaced the servo, no improvement.
I had discounted the reaction valve piston as I had lubed it with red rubber grease a year ago.
However I thought I’ll have another go at lubing the piston as it is the easiest thing to do. This time I used white lithium grease.
Success, a nice hot drive and not a hint of brakes dragging.
The thing i can’t get my head around is that the brakes were perfect when cold but the reaction valve piston only started sticking when things heated up.
So pleased to have a fix.
Cheers
Mark
Mark Brown
1971 S3 Etype
Son: when I grow up, I want to be a pilot
Dad: You’ll have to choose.

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#16 Re: Dragging brakes.

Post by Tom W » Fri Nov 08, 2019 6:14 am

Glad you’ve fixed it.

Mine was doing the same, fine when cold/warm only a problem on the very hottest days. I think as things heat up the differential expansion of the different materials causes sticking where there was no sticking before.

Is the lithium grease you used compatible with brake fluid? I’ve use red rubber grease before, and many rebuild kits come with a little sachet of some sort of silicon grease, but I’ve never heard of using lithium grease on hydraulic systems.
Tom
1970 S2 FHC

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#17 Re: Dragging brakes.

Post by 42south » Sat Nov 09, 2019 5:23 am

Thanks Tom

I’ll check out the grease thing. I’m going to polish up the little piston and the bore with some 2000 grit wet and dry sandpaper.

Cheers
Mark
Mark Brown
1971 S3 Etype
Son: when I grow up, I want to be a pilot
Dad: You’ll have to choose.

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#18 Re: Dragging brakes.

Post by Tom W » Sat Nov 09, 2019 9:24 am

Polishing the piston won’t make a difference unless it’s severely rusty, but it won’t harm either. The piston is a clearance fit in the bore, it’s the rubber seal around it that does the sealing / sticking. The pitting on my master cylinder was only visible with the master cylinder removed from the car and a torch shining down into the bore. It wasn’t much, but it must have been enough for the lip seal to catch on when returning.
Tom
1970 S2 FHC

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