Brake pedal slowly drops to floor under pressure

[CJH]

A new little 'feature' of my braking system appeared today. If I keep my foot on the brake pedal while the engine is running it slowly drops to the floor. The pedal is firm initially, and braking efficiency is fine while I'm driving along and only using them briefly as you do under normal driving, but I'm sure that if I had to apply them for longer then I'd have no pedal left. Without the engine running the pedal does the same thing, but it takes much longer to drop.

There are no signs of any leaks, and the level in the fluid reservoir is fine. Rear brakes were overhauled last year, including new cylinders and the whole system was flushed through with new fluid. One other possible clue - I went on a short trip to the shops and the brake warning light stayed on when I released the handbrake. I assumed a faulty handbrake switch, but then I started to notice the pedal behaviour. On the return trip the light went out as normal, but the brake pedal was still doing the same.

So where do I start?

[mark]

I would start at the master cylinder with the plunger seal being the culprit, that is if your sure you've not got a leak somewhere

mark

[CJH]

Thank you - I'll have a more thorough look for leaks. If the plunger seal isn't sealing, is it worth replacing it, or should I get a replacement master cylinder?

Any idea what triggers the brake warning light - how does it know there's a fault?

[CJH]

No sign of any leaks around the master cylinder either. But for £38 from GSF (using this weekend's SUN35 discount code) I think I'll just replace it and see if that fixes the problem. They also do a service kit which works out at half that price, but I think that's something to consider when you know whether it's serviceable. I'm in a bit of a rush to fix it since I plan to go away in the van next weekend.

[mark]

I would have thought that there will be pitting inside cylinder so you would only ever get a short term fix with the service kit.......when I changed mine brickwerks where selling vag ones with the part number ground off...... had a quick butchers and they're out of stock...
without actually looking at the problem my first point of call would be to change the master cylinder and if the warning light continues then start looking for electrical problems

mark

[CJH]

I'm glad you said that - I've just finished fitting my new GSF cylinder. Just a fun evening of bleeding all the hydraulics to look forward to tomorrow.

The one I removed is an original one, so I'll more than likely take it apart to see if it's worth servicing. Mind you, if it's THE original one, it probably won't be.

[CJH]

The new master cylinder seems to have fixed the dropping pedal. After a wasted evening yesterday with a Gunson's Messi-Bleed, this evening it took half an hour to bleed with a willing pedal operative. Has anyone found a good way to bleed brakes single-handed? The Gunson's system uses a spare tyre to pressurise the reservoir, but I couldn't get the cap to seal, and it was a huge amount of faff. I've seem vacuum pumps that connect to the bleed nipple, but it looks like they can be ambiguous because they draw air in around the bleed nipple thread. A non-return valve on the plastic hose seems like a good idea, but again I think you can get air drawn back in around the screw threads. An operative on the pedal seems easier, cheaper, and more efficient I think.

I'm not convinced the pedal is as solid as I remember though, so I'll probably need to give the bleeding another go, but the original problem seems to have been fixed, and braking seems perfectly efficient.

Thanks for the pointer.

[jimrat]

Gunson worked ok for me. It's messy if you fill up the Gunson bottle with brake fluid, but if you skip that bit then it works fine. Just keep your eye on the fluid level in the reservoir and add extra if needed. I use the wheel off my son's bike to provide the pressure - it's much smaller than a van tyre, and holds less pressure (i.e. less likely to blow lid off reservoir = BIG mess).

Agree, vacuum type are carp. I bought an expensive Sealey one but couldn't get on with it.

Cheers
Jim

[Oldiebut goodie]

Yep -= don't get a vacuum type - as said it draws air in around the threads making it useless! Tried mine on several vehicles, exactly the same. Also useless on my Merc due to some valve in the brake lines which stops it working!

[CJH]

Gunson worked ok for me. It's messy if you fill up the Gunson bottle with brake fluid, but if you skip that bit then it works fine.

That's a good idea - I might try that the next time my pedal operative isn't home.

I use the wheel off my son's bike to provide the pressure - it's much smaller than a van tyre, and holds less pressure (i.e. less likely to blow lid off reservoir = BIG mess).

I used the space-saver spare out of my son's Polo, and it fitted on the driver's seat. I let it down to 20psi as per the instructions, but I was surprised to find I had to keep refilling it. I would imagine a bike tyre barely holds enough to bleed one corner.

[Dazco]

I use a length of tubing into a jam jar through a hole punched in the lid and the jar maybe a third full of brake fluid . Just crack the bleed nipple open so there is still pressure on the pedal . Couple of pumps on the brake peddle soon fills the hose .
I use a piece of wood jammed between the steering wheel and brake pedal to keep it depressed while I go under to nip the bleed nipple up .
Much easier to get Andrea out of the house though to help

[CJH]

I'm not convinced the pedal is as solid as I remember though, so I'll probably need to give the bleeding another go, but the original problem seems to have been fixed, and braking seems perfectly efficient.

I had another go at bleeding the brakes again today, and the pedal is now nice and solid. The key seemed to be the upper bleed nipple on the front right calliper. Mine's got the early ATE callipers, with the two bleed nipples. I had done the lower one, but I guess the upper one is where air will gather. In fact I hit a rich seam of air - it just kept forcing bubbles out for ages. I got through a good litre of fluid just on that calliper. I don't know whether these callipers are known for trapping a lot of air, or perhaps there's a leaky seal and it's time I serviced these callipers.

Once I'd got the pedal/bleeding finished, I fired up the engine and the warning light was back! So thinking I might have a leaky calliper I've been scratching around all day looking for somewhere that's got callipers, discs, pads, bearings, backing plates all in stock (with no joy I might add), but eventually I pulled the multiplug off the back of the warning light and pushed it back on, and bingo, the light works properly now!

[California Dreamin]

I've a proper metal screw cap that replaces the reservoir top. This has an air line adapter which connects to my compressor via a 2-5 psi regulator. Works a treat, you just have to remember to top up after each wheel bleed. The beauty is that there isn't any possibility of damage to the seals riding over the wear ridge, or bubbles being created by heavy piston movement.
You know where I am Chris.

Martin

[Wychall]

Just replaced a couple of front pipes and bled the system single handed with no problems. My technique is to add a 60ml surgical syringe on to the end of a bleed tube. Fit to nipple, open nipple then go push that pedal. Pump as needed, remembering to check the syringe contents every half dozen pumps. Syringe readily available on eBay.