DISAPPEARING BRAKE PEDAL

[EXKPeter]

Hi, New to this and posted already (wrongly I think) on the S1 board. Yesterday the brake pedal disappeared to the floor suddenly and frantic pumping restored pressure enough when needing to slow down to limp cautiously back home. After 15 minutes or so, as if by magic, the brakes decided to go back to normal. No loss of brake fluid noted.All is now normal except lack of confidence. Anyone experienced this before I embark on major replacement ( servo,master cylinder, reaction vv ). Not knowing what the defect was in the first place it will be difficult to know if the problem is fixed.
Thanks Peter
1965 4.2 FHC

[Gfhug]

Peter, posting here or on the S1 doesn't matter. Maybe no-one has responded because it's a problem not seen before?
I can't comment lots about it but will ask you to post a bit more information, please.
How long have you had the car and how many miles, especially recent miles? In that has the car been laid up for a while, a daily driver, you're new to E Types or lots of experience.
If you've only had the car a short while after a long time of little use maybe there is a sticky seal or valve. I'm only guessing. But if you can give us some more information it may help the wise people where to start their suggestions.

Sorry I can't be more helpful. This forum is an excellent place to seek knowledge and help, let's hope someone else can come to your rescue.

Geoff

[Heuer]

My guess would be air in the system. Suggest you start by fully bleeding the system using new brake fluid (it should be changed every two years anyway).

[Geoff Green]

My first thought is high moisture content brake fluid boiling at a much lower temperature than fresh fluid. This creates a gas area displacing the fluid. Gas compresses and fluid doesn’t. After cooling off from a short drive without using the brakes the gas condenses back to fluid and brakes work normally.

Causes are hard braking, stuck piston holding the brake pad to the rotor, high ambient driving ( Sahara Desert in the summer). The braking heat on good fluid will not boil however add some moisture and the pedal goes to the floor.

[Mark Gordon]

It does sound like hot brake fluid, but the question then becomes why did the fluid get hot enough to boil? I'd be looking for a sticking brake piston that causes the pad to drag on the rotor. Or possibly a sticking piston in the front of the MC which is keeping the reaction valve from returning atmospheric pressure to the vacuum booster once the brake pedal is released.

[EXKPeter]

Thanks for the interest and responses. I have owned the car for 14 years and she is a veteran of quite a few English and Continental rallies etc and gets driven reasonably often, even if only a quick blat around the local lanes to warm everything through. I have thought about the air in fluid suggestion but it felt a bit more 'digital' than that-everything fine then nothing-and then back to fine again. Boiling fluid seems unlikely in an English summer, no evidence of brake drag and I was just gently pottering to the Bicester Scramble so no great stress on the brakes.Mind you I am sure the brake fluid could do with renewal so thank you for the reminder.
I am leaning toward a seal type problem. If the reaction valve hung up presumably the servo boost would not kick in but would I lose all braking as I would have thought that the unboosted pressure from the master cylinder would at least retain some braking ? At the moment I am thinking it is more likely to be a master cylinder issue where, because of a seal problem I am not generating any fluid pressure to start the process.As the master cylinder has not been repaired/replaced during my ownership it would not be wasted if wrong but I am slightly alarmed by the posts about 'bleeding problems' ! If it is that bad it may be sensible to overhaul the servo at the same time as the MC albeit more cost and cracked knuckles. Thanks everyone,and further suggestions always welcomed.
Peter

[Heuer]

Loss of servo would not mean you lose braking - it is servo assistance not servo control.

If it is a seal problem you will find the brake fluid will be grey or black when you bleed as the rubber disintegrates. There are also two separate brake circuits so if one fails the other should still work. I say again, the first test is to replace the fluid (£30 for 5L from Halfords) because if you start pulling things apart before diagnosing the issue you will not know where to look or if you have solved it.

Pedal to the floor without any loss of fluid and then returning to normal is extremely odd. It can only be what connects the pedal to the pads. Next would be to o/h the m/c. Servo would be the last thing to overhaul. Incidentally do you have the servo heat shield C26500 that was installed on all 4.2's from around 1965? Servo overheating can cause problems and Jaguar recommended reftrofitting it to earlier cars. SNGB sell them: https://www.sngbarratt.com/uk/#!/Englis ... 761b61f154



Fosseway sell one if you can't get the original style: http://www.fossewayperformance.co.uk/servo-heat-shield/

[EXKPeter]

David, Most helpful thank you. I agree on the mc diagnosis and will fit a new one.I do not think I have the heat shield for the servo so thanks for the tip. All good stuff so thanks forum members.
Peter

[mtnjag]

Very similar problem on my 62 3.8 with the original bellows/dual masters setup. Wilwood Dynalites on front and 4.2 calipers on rear. Many miles on car with this set up. Last changed fluid with Dot 4 about 4 years ago. Slightly larger masters to accommodate the wilwoods. Heat shield on reservoirs. No leaks. No fluid use. Tried pedal upon getting home and it didn’t sink so thinking no by-pass.
Driving for 30 miles or so with normal braking. Long mellow up hill cruise in cool weather with little brake use. Crested the hill in a higher gear with lower rpm and gently applied brakes to slow and the pedal went to floor! Was able to pump them up, and they felt soft but then went to normal. Nervous drive home constantly questioning brakes.

Going to change fluid but don’t want to tear the system apart for no reason. System dates close to 25 years ago when I upgraded. Live in central California and car is driven regularly.

Thought I read about this in Definitive Etype but can’t find it.

Thoughts?

[mtnjag]

Popped boots off back of MCs and no fluid is present so again not suspecting a bypass of the MC seal. All joints appear dry and see no fluid indicators on calipers. No loss of fluid in reservoir.

Anyone?

[mgcjag]

Hi...if you havent replaced your fluid in 4 years there is possibly a clue...especially how pumping brings pedal back to firm....replace fluid and bleed.....probably moisture and air in the mix.....Steve

[mtnjag]

Yep, happening tomorrow.

[mtnjag]

Dirty fluid with air, especially in rear system. Slightly wet around supply hose from reservoir to MC feed hard line. Fully bled new fluid thru system and replaced supply hose. Brakes Felt ok before, feels better now. Hope that does it. Nice to find something.

[dal2.0litrefrogeye]

Had this on my S 1 yonks back , check the rear brake calipers , one side of mine was sticking on slightly, then warming up and letting by enough for the pedal to floor , let it all cool down and all was well , hands up I replaced the master cylinder 1st thinking it was that starting to fail , ( same symptoms ) but defo have a look at rear brake calipers look for the seals getting hot or starting to fail

[mtnjag]

Up on jacks right now doing a clutch slave cylinder so I’ll take a look. Thinking this is hard to determine in the garage with cold brakes. How did you diagnose the problem?
Thanks

[mtnjag]

Update, no repeat performance since full renew and bleed.

[Allrand]

If the reaction valve sticks (doesn't react) fluid is simply pumped back into the reservoir hence no evidence of leaks. Rapid pumping will unjam the it and restoore braking. Check for possible leak from overflowing st reservoir.