Slightly vague brakes

[265bhp]

Hi guys

Apologies in advance as this question is a little bit vague, but I’m not quite sure how else to describe the predicament

Let’s just say that yesterday when I took the car out ….after many weeks of lying idle all seem to be okay as I popped down to the petrol station to get more fuel and return home …looking forward to an early drive this morning with no hiccups

All I can say is that as soon as I set off the brakes felt vague

As I continued driving, I tried to think what the issue might be, I had possibly got some WD-40 onto the brake discs yesterday as I needed to clean the wheels, but after an hour of driving, nothing seems to be improving
For all sorts of reasons I could only than check the car over later today and it’s now too late to go back out for a drive, but I’ve wiped the surface of the discs with brake cleaner and I have checked all of the pipes and the connections, including the low-pressure rubber hoses from the reservoirs, and I have no leaks anywhere

I thought the bellows servo unit on the early cars was assisted once the engine was running, and things would feel different as opposed to simply pressing the brake pedal when the engine was switched off… Perhaps I am mistaken, but in any case there is no real difference.

it doesn’t feel as though there’s air in the system (I need to test this I guess) because repeated pumping of the brake pedal doesn’t seem to create a greater sense of resistance.

I just wondered if there’s something that might have happened with either

a) The servo unit or
B) the plungers inside the master cylinders, have partially stuck somewhere down the cylinders… (not quite sure if that makes any sense at all…)

Anyway, would appreciate thoughts please.

Aside from that it was wonderful being out and about in the car again….

Many Thanks
Jonathan

[Simonpfhc]

Hi Jonathan,

You can check that the servo is allowing vacuum into the bellows by listening to the revs at idle when you apply the brakes. Can’t remember if it drops or rises, but the revs should change as the engine is robbed of some vacuum.

[265bhp]

Thanks Simon….I think I can try that in the garage later this evening….

[265bhp]

Right….tried that….made absolutely no difference whatsoever….

What do I do next…please

[Simonpfhc]

So now you need to start a bit of servo fault diagnosis. I would start with removing the vacuum hose and listen for immediate hissing as the reservac should still be holding some vacuum. Then run the engine and check you have vacuum at the end of that removed hose.

If that is all good, then either the bellows aren’t holding vacuum or the valve is not being operated to allow vacuum.

[Jack the lad]

A quick test of the servo operation without any dismantling is to empty the vacuum reservoir by pumping the brake pedal several times, then put pressure on the foot brake pedal as you start the engine. If the servo and its associated parts are working the pedal should move towards to floor slightly.

[265bhp]

Thanks both…testing will resume this evening..!

[malcolm]

I had a blocked vacuum hose. It had degenerated over time and had sort of collapsed in the middle. Make sure it's clear perhaps?

[265bhp]

Evening all

I’ve done some testing… With the engine running if I depress the brake pedal, there is absolutely no change in the revs whatsoever.

Having run the engine for about a minute and then cracked off the vacuum hose from the top of the bellows unit there was absolutely no hiss of escaping at all ….no build up of vacuum

With the vacuum hose still disconnected from the bellows unit, I restarted the engine and there was a very strong and audible hiss and a decent amount of suction on the end of the pipe when I placed my finger over it.

Having a look at the little valve in the front face of the bellows cover plate. I took these two pictures as I was interested to see what happened to this thing as soon as the brake pedal was depressed… It doesn’t really look like it’s doing terribly much moving which makes me wonder have I got the adjusting arm in the wrong place completely….and it isn’t allowing a vacuum to build up at all….?

Do these pictures look how it should all do I need to be adjusting the lever ….?….the first picture is at rest….the second is with the pedal fully depressed

Many thanks

Jonathan

[Simonpfhc]

Well, it does look like that valve isn’t being activated.

A couple of observations;

Firstly, there is a setup procedure in the service manual which is pretty straight forward to get it correctly set up. You just need to loosen that castellated nut (with the spring attached) and adjust accordingly.

However, I doubt that has changed overnight causing your problem.

Secondly, it is quite easy to test - just use a thin screwdriver and activate that button and (with the engine running) the bellows should be easy to just push in.

I still think WD40 on the pads is a strong culprit.

[Dave K]

The easiest method by far is to make up a fitting for the servo and put a vacuum gauge on it, you can test the vacuum and also adjust at the same time.
I have all you need if you want to borrow it let me know.

Dave

[265bhp]

Thanks Dave

Have sent you a PM

All the best
Jonathan

[MarekH]

Not having vacuum assist simply means you have to push very hard to brake. That's not anything I'd describe as "vague" - it's pretty definite and extremely scary when you first encounter it. If you think you have a vacuum problem, just disable it and see whether your brakes feel any different. (Don't do that out on the main road - the side road outside your house will do.)

kind regards
Marek

[265bhp]

Hi Marek

…vague was the way in which the car would come to a halt…I am used to no servo at all on my ‘74 911…but at least hard pressure results in it stopping…the E was more like a gradual sense of slowing down with the pedal applied…

It’s interesting to check the servo functioning so I will continue to do that, but I get your point about trying it when disconnected….I will try that Saturday as well

Best
Jonathan

[Simonpfhc]

You also need to check/replace your non-return valve on the reservac as it’s obviously not working.

As Merak is suggesting, I think all these possible issues are red herrings and not causing your ‘vague’ braking. My money is still on WD40 on the pads

[DWW]

It is possible that the WD40 is now bedded in on the pads rendering then no more, deceased....that is what I once heard somewhere but never tried.

[MarekH]

…the E was more like a gradual sense of slowing down with the pedal applied…

Remind me:- do you have two independent brake circuits? Where I am going with this is does one work and the other only half work for some reason?

A non-working non-return valve ought not be the end of the world. You are generating plenty of vacuum whenever the engine is running and the throttle is closed. Vacuum only gets "used up" when you come off of the brakes and there ought to be enough in the tank for four or five assists.

I would have thought having a potentially always open connection to the outside ought to result in a higher idle and you haven't reported that.

I might be free at the weekend, tree pruning and horses permitting.

kind regards
Marek

[abowie]

You also need to check/replace your non-return valve on the reservac as it’s obviously not working.

Looking at the picture, I wonder if the valve in the bellows is always actuated and so there is a constant slow vac leak.

There should be a gap between the arm and the button.



I agree that vacuum is likely a red herring here. Back in the day we used to "clean" brakes shoes contaminated with brake fluid by pouring petrol on them and lighting it.

[dlgis]

Surely if there is any doubt as to whether you have contaminated your brakes it’s worth replacing the pads just to be sure Also when i got my etype 18 months ago i found the braking to be more 1960s and casual than i could live with, yes it would stop but if a modern car did an emergency brake in front I’d be in trouble. 4 pot front brakes have significantly improved my confidence in them

[DWW]