Reoccuring air in brake lines

[pointynoggin]

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

[BigVWman]

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!

[Wayland]

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.

[pointynoggin]

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?

[smutts]

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.

[R.O.R-2.0]

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..

[pointynoggin]

That is what i thought i would hear but hoping not to.

Is there a way to test it?

[R.O.R-2.0]

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..

[the caveman]

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.

[pointynoggin]

Were the leaks in the connections?

Only one with air is rear drivers.

[the caveman]

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

[smutts]

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.

[R.O.R-2.0]

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..

[damac]

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.

[R.O.R-2.0]

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..