Brake light warning lamp coming on then out again

[Mysterae]

Your opinion on this one please.

The warning lamp for the brakes is coming on briefly then going out in the following situation:

With the engine running, handbrake off, when I press the brake pedal the brake warning lamp comes on the goes out a few seconds later. Press the brake pedal again, comes on again and out just like before. I have just finished refurbing my front brakes but it did this before I touched them. At first I thought it may have been the low brake fluid switch but it's not.

So a likely candidate is the low vacuum switch. What I'm not so sure of if it is the switch or an actual low vacuum situation, and if so what's causing it and how can I fault it? I tested the servo as per the manual (engine off, press press brake pedal to remove vacuum, keep pressed and start engine, brake pedal gradually drops) and according to the manual that's okay.

All help appreciated!

[dodge1979]

When you depress.the pedal and the light shows, are you testing this in motion or when stationary...ie on your driveway? If so, is.this with or without the handbrake applied.

[Mysterae]

Hi Darren, it's stationary with the handbrake off, but I remember it doing it while moving. Don't need to press the pedal that hard, just it's full travel.

[Mysterae]

I've done a bit of research and have a few things to look at tomorrow:

- to confirm it is a vacuum problem I'll disconnect the vacuum switch on the brake servo and short it out (the cable, not the switch). If the light doesn't come on when I press the brake pedal then that will confirm it's a vacuum problem. I'm 99% sure the brake servo is okay as it passess the test from the service manual (see first post).

- if it is a vacuum problem then it's time to check the vacuum lines first.

- as I don't have the EBPV anymore the vacuum line that used to go to the EBPV has a screw in the end of the pipe, which is coiled up under the boot floor. Not the best option (wasn't me!) so I think I'll shorten that pipe and close it off 100mm from the switch in the compartment. This is my first port of call.

- then check the pipe going to the between the throttle jack valve and actuator (black and red pipe I think).

- get my head under the dash and make sure all the vacuum lines are connected, may need to access the actuators for the flaps to make sure.

- if all the vacuum lines are okay then it must be the vacuum pump that is no longer providing enough suck. That would suck, if you know what I mean!

- an outside shot is to check the brake lights for a bad connection.

Interestingly, in my research I have found that some cars have as standard and some folk retro fit is a vacuum tank reservoir. One idea is to use an empty and clean fire extinguisher bottle as the reservoir. A 600g or 1kg bottle provides enough in reserve. They do this for a number of reasons:

- more vacuum in reserve means the clutch of the vacuum pump doesn't engage/dis-engage as often so less wear, (it will engage for longer though to build up the extra vacuum),

- so they can add other vacuum using hardware such as a servo assisted clutch (now that would be a good idea on the Esprit!),

- in the case of an engine stall, there is enough vacuum in reserve for the brake servo assist to work for a bit longer.

Anyone tried this, think it's a good/bad idea etc?!

[Mysterae]

I've not had the time to do much faulting recently, but I have done some pondering!

I was thinking about adding more vacuum to the system and see if the light still comes on while pressing the brake pedal. I don't think it would cause any problems?

Is the vacuum pump on the Esprit clutch operated or is it active all the time?

[Vulcan Grey]

The mechanical vacuum pump on the cars without power steering is active all the time.

[Mysterae]

Thanks for the confirmation Travis, I wasn't sure. What happens when there is enough vacuum? Is there something in the vac pump that releases or expels?

I am considering making a vac reservoir out of an empty fire extinguisher to help the system, but then again it's not needed on a standard car.

Also, can someone do me a favour? Can you post a picture of the brake servo where the shaft from the brake pedal meets? There is a filter at that point (which is meant to be cleaned routinely) and I'm not sure if mine is placed correctly.

[Mysterae]

No one got a photo of the brake servo

[J400GED]

Have you sorted your vacuum issue?

If not, first place to check is for the heater/ventilation flap actuators, one quick check is, with the engine running, to cycle the ventilation from screen to fresh air to recirc and see if you can hear the flaps cycling.then cycle it to fresh air and the inlet flap under the bonnet should open, then stay open when you turn off the engine.

Then check the hoses are connected as these can fall off. This bit can be a pain as they are located behind the centre console.

Hope this helps,

Ged

[Mysterae]

Cheers Ged. I've not sorted the problem sadly. Not that it's proving elusive, just haven't had the time to fault it. That is a good call on the ventilation flaps, I'll check that when I get the chance - don't know when that will be though. That's the problem with not having a proper garage to work in, I'm at the mercy of the weather and daylight!

[MikieP]

Just a note - I used to use the screw in the hose fix but found that 1) the hose expanded and 2) the screw lost its sealing fit, and there was leakage. Use clamp on the hose and see if things change once clamped. Seal the hose with goop....

[Mysterae]

Amongst other jobs this weekend, this was one I tried to tackle but still haven't got to the bottom of it. As a reminder, the brake light comes on when I press the brake pedal, then goes out again after 1 to 2 seconds, being triggered by the low vacuum sensor.

 

Here's what I've done and or found out.

 

- Mucked about with the ventilation controls and they are moving, but no change to the problem.

 

- Disconnected the vacuum feed to the heater controls and blanked off it's feed. Obviously the heater controls no longer worked, but the problem was still present. So I reckon I can rule any issue in the heater controls area. I had a look under the dash anyway and didn't see anything untoward. 

 

- I disconnected the other feed for vacuum as well as the heater controls, blanked off and the problem is still evident. 

 

Here's something new - with the engine running at steady 2600 rpm, the brake light doesn't light when the brake pedal is pressed. Is the vacuum pump load dependant? i.e. if the engine is running faster, there's more vacuum (as it's belt driven)? Is this a sign of an ageing vacuum pump? As the belt hasn't been changed in a long time (6 years, it's been off the road) is the belt slipping and not turning the vacuum pump enough? Is the vacuum pressure sensor a bit to sensitive? 

 

I checked out the brake servo and watched it while the other half pressed the pedal. Not having anything to compare it to I'm unaware if the noise it makes is normal. Is there a refurb kit for the servo unit? Do we know what other car it's from?

 

! I just re-read one of my posts and there are a few things there that I'm yet to try (brake switch being one of them). Bad earth from the brake lights? 

 

All help welcome!

[Mysterae]

Does anyone know at what vacuum pressure the vacuum sensor operates at?

 

I've got a vacuum gauge coming to me in the post and need to correlate what pressure I have and when the sensor should operate. If it is the sensor that's just as worse as I can't find a replacement (or servo unit) anywhere.

 

Anyone fitted one from another model of car?

[Mysterae]

Feel like I'm talking to myself here, replying to my own reply, but any ways...!

 

Made a bit of progress today.

 

- Removed the check valve from the brake servo and it's fine (allows flow one direction but not the other)

 

- Disconnected the brake fluid level sensor and the brake warning light still comes on when the brake pedal is pressed, so it's not that.

 

- Disconnected the vacuum sensor and the brake warning light doesn't come on when the brake pedal is pressed, so this is the symptom not necessarily the cause.

 

- Connected up a vacuum gauge to the vacuum circuit with the brake servo out of the vacuum circuit and I'm getting about 7.5 inHg (see the picture below), which is equivalent to 0.254 bar or 3.7 psi. This doesn't seem like a lot!

 

 

- With the brake servo in circuit with the vacuum gauge I can see the vacuum drop when the brake pedal is pressed. It drops to about 2.5 inHg per press of the brakes, and if I constantly press, lift, press lift etc I can get the vacuum to be almost nothing. The vacuum always builds back up again, and does increase slightly with engine revs.

 

- While doing the above, the vacuum sensor is triggered just below 5 inHg. No idea if this is correct operation or not.

 

- To improve the vacuum, for a laugh as I knew it would happen, I connected up a small plastic bottle to increase the volume of the vacuum. As predicted it sucked in the bottle and held the vacuum (see picture) but made little difference to the fault.

 

 

- To improve the vacuum, I connected up my compressor and the fluid bleed kit as it creates a vacuum enough to suck through brake fluid. I didn't have the vacuum gauge in circuit unfortunately as it would have been interesting to see what vacuum I had with both the car and the bleed kit sucking away. Anyway, the extra vacuum improved things a little, but I could still get the vacuum sensor to trigger after multiple presses of the brake pedal, but the brake pedal did feel better (could have been my imagination). The brake warning light was quicker to go out, obviously meaning the vacuum was getting back up to 5 inHg quicker.

 

So, what next? I think it is one (or more!) of 3 possible issues:

 

A. The vacuum sensor is worn and or become over sensitive and needs to be replaced?

B. The vacuum pump isn't as efficient as it once was and needs replacing?

C. The brake servo is releasing to much vacuum than it should when depressed and should replaced?

 

Any one any ideas?

 

 

 

[Mysterae]

For completeness, I thought I'd update this thread.

 

The answer was B, the vacuum pump was shot!

 

Before I replaced the vacuum pump the vacuum pressure I had was around 7 inHg, see a few pics back.

 

Now that I have replaced the vacuum pump with a new unit, the vacuum pressure is now just over 25 inHg, quite a difference!

 

 

More importantly, the brake tell tale warning light that was coming on after pressing the brake twice or more, is no longer lighting. Problem solved .