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General Information => Troubleshooting => Topic started by: pointynoggin on October 02, 2011, 06:08:45 am

Title: Reoccuring air in brake lines
Post by: pointynoggin on October 02, 2011, 06:08:45 am: I have a 91 jetta.

I have been dealing with soft brakes for a while now. I bleed them and they are good for a day then soft again.

Last time I bleed them I couldn't get all the air out of the rear brakes, each peddle push brought bubbles out at the bottom of the stroke.

I have no fluid leaks so I think it must be a 'check valve' type of leak.

Can anyone suggest if it is the proportional valve or the master cylinder or line connections?

Thanks
Title: Re: Reoccuring air in brake lines
Post by: BigVWman on October 02, 2011, 06:34:25 am: How old is the fluid? Have you bled it all to new? Are the brakes dragging? Old fluid becomes water contaminated and it lowers the boiling point to where hard braking or dragging brakes can boil the fluid creating vapor issues. Most brake fluids exccept some of the newer synthetics are hygroscopic and that means they absorb water as they age!
Title: Re: Reoccuring air in brake lines
Post by: Wayland on October 02, 2011, 07:46:46 pm: Not sure if this can happen with a wheel cylinder, but I once had a hydraulic clutch slave that didn't leak fluid out, but would suck a tiny amount of air in every time the pedal was released. The only way I was able to find it was by having someone else press and release the pedal while I listened near the cylinder.
Title: Re: Reoccuring air in brake lines
Post by: pointynoggin on October 02, 2011, 09:41:02 pm: the fluid is good.

The leaking cylinder might be. The bubbles come at the last 20% of the stroke and the bleed is at the top of the cylinder so I don't think so.

I've an idea to get reservoir cap from the wrecker put a air fitting in it and do a low pressure bleed, than a peddle pump bleed to locate how far up the line the bubbles are coming in.

any better ideas?
Title: Re: Reoccuring air in brake lines
Post by: smutts on October 03, 2011, 02:27:22 pm: You have my sympathies, as I have the same pain in the arse. No fluid leak, but air getting into one of the dual circuits now and then. I replaced the master cylinder, but I don't think that has sorted it, so rear wheel cylinder is the next suspect. Good Luck.
Title: Re: Reoccuring air in brake lines
Post by: R.O.R-2.0 on October 05, 2011, 01:55:14 pm: i would be looking at the prop valve.. my GTI used to get air in the lines almost daily till i changed out my prop valve.. they ARE SPENDY, but they usually fix things..
Title: Re: Reoccuring air in brake lines
Post by: pointynoggin on October 06, 2011, 09:14:07 pm: That is what i thought i would hear but hoping not to.

Is there a way to test it?
Title: Re: Reoccuring air in brake lines
Post by: R.O.R-2.0 on October 07, 2011, 09:54:16 am: Quote from: pointynoggin on October 06, 2011, 09:14:07 pm
That is what i thought i would hear but hoping not to.

Is there a way to test it?

i never got to test mine, it was leaking, and i needed to replace it. i temp fixed the leaky prop valve with split shots (round lead fishing weights) stuck in the inlet side of the prop valve.. then i stuck the brake lines back in the prop valve, with the split shots in the prop valve already, then tightened the lines down on the split shots.. it stopped my leak until i got another prop valve.. and people say that you cant use an ABS prop valve in a non-abs car, but dont believe a word of it, the abs prop valve works just fine, maybe a little better even. seems like it biases the rears a little more than a normal prop valve..
Title: Re: Reoccuring air in brake lines
Post by: the caveman on October 09, 2011, 06:37:49 pm: I've seen at least 2 cars [both mk2's !] that had a line leaking backwards. That is, they didn't leak fluid, but let enough air in that it couldn't even let me be bleed either side in the rear properly. I only found it by blocking off each circuit and then testing. The first one was a real pain to find.
Title: Re: Reoccuring air in brake lines
Post by: pointynoggin on October 10, 2011, 09:45:23 pm: Were the leaks in the connections?

Only one with air is rear drivers.
Title: Re: Reoccuring air in brake lines
Post by: the caveman on October 11, 2011, 04:41:00 pm: Yes we figured the leak was at the connections. I can't say it was the flaring ,otherwise it would have leaked from new. I do remember one lines was the one that ran under the seat from the valve to the r/s
Title: Re: Reoccuring air in brake lines
Post by: smutts on October 19, 2011, 08:31:45 am: How the hell does it NOT leak fluid at 1000psi or so, yet air at atmospheric pressure 15psi waltzes straight into the rear pipework? It is a weekly bleed job at the moment. I will let you know how I get on with mine. ::)
Title: Re: Reoccuring air in brake lines
Post by: R.O.R-2.0 on October 19, 2011, 08:58:16 am: Quote from: smutts on October 19, 2011, 08:31:45 am
How the hell does it NOT leak fluid at 1000psi or so, yet air at atmospheric pressure 15psi waltzes straight into the rear pipework? It is a weekly bleed job at the moment. I will let you know how I get on with mine. ::)

my thoughts exactly..
Title: Re: Reoccuring air in brake lines
Post by: damac on October 21, 2011, 11:37:02 pm: I was wondering about that prop valve in my 85, I bled it like it said and zip tied it to the proper position and did them in order, etc. I replaced all the brake parts except that prop valve, hard lines and master. Never seen an external leak or lose fluid from reservoir but I can not stop the air bubbles. I have attacked it with a air compressor bleeder. I even gravity bled for hours at a time on each port. My last effort was going to be using a spare master lid and attach an air hose and put low pressure through and push the fluid through slowly. You can attach something to catch the moisture in the air hose before attaching to the reservoir.

I have ended up just letting it be but I wanted to replace that valve to see what is up.
Title: Re: Reoccuring air in brake lines
Post by: R.O.R-2.0 on October 22, 2011, 02:41:14 pm: i bet anything its sucking past the seals in the prop valve when you let off the brakes.. my car used to get air in the rears before i changed the prop valve..
Title: Re: Reoccuring air in brake lines
Post by: smutts on October 26, 2011, 03:16:22 pm: Quote
i bet anything its sucking past the seals in the prop valve
So no chance of it being the (much!) cheaper wheel cylinder......... :( :(
Title: Re: Reoccuring air in brake lines
Post by: R.O.R-2.0 on October 26, 2011, 04:31:55 pm: Quote from: smutts on October 26, 2011, 03:16:22 pm
Quote
i bet anything its sucking past the seals in the prop valve
So no chance of it being the (much!) cheaper wheel cylinder......... :( :(

every issue ive had was from the prop valve..
Title: Re: Reoccuring air in brake lines
Post by: smutts on November 23, 2011, 11:58:41 am: Well, just to be awkward, the brakes have decided to behave themselves the last few weeks, but as I do a lot of driving over Dartmoor with lots of 25% grades, bends, stone walls, wild horses and the sheep like to sleep on the road at night, it will just be a matter of time............. ;D
Title: Re: Reoccuring air in brake lines
Post by: pointynoggin on June 10, 2012, 08:25:00 am: Well i changed the master cylinder and the prop valve with no effect. Guess I will change the rear pistons and pads now.

One other note, when i press,the peddle during bleeding it clicks halfway down, loudly if i am pressing hard. What is the deal with that?
Title: Re: Reoccuring air in brake lines
Post by: smutts on June 10, 2012, 11:00:34 am: Quote
when i press,the peddle during bleeding it clicks halfway down, loudly if i am pressing hard. What is the deal with that?

Tandem master cylinders do that. The fluid pressure from your foot pressing the pedal presses fluid into one circuit, and the pressure also pushes a dumb piston that pressures the second circuit. The sound is the foot piston clunking against the dumb piston, as the fluid that usually passes the force is flying out of your bleed nipple. Clever, because if your pipe bursts, once the fluid has gone, you still have some brakes. My very old land rover doesn't have this and will simply carry on to the accident. :o
Title: Re: Reoccuring air in brake lines
Post by: pointynoggin on June 16, 2012, 09:43:13 pm: thanks, that is good to know :)