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Old 10-08-2015, 08:08 AM   #337
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As far as I can tell (and this is coming from someone that is having to get someone else to fix it!). There is an expected pressure difference between front and rear circuits (determined by a couple of spring loaded valves that let flow to front (via the main metering valve), and to rear). The switch sits in a chamber between the two circuits and looks for pressure changes associated with the two circuits being in balance (just with different pressures being applied).



However if the switch is activated it could be electrical (wiring or switch), the combination valve itself (i.e. the springs not opening at the appropriate time), or a leak on either circuit (so they aren't reaching pressure and causing a differential).

It is pretty crude. I am told that the rating of the metering valve spring matched to the type of brakes installed - disc front/drum rear what more pressure going to the front vs disc all around want less pressure to the front. Which means buying the correct replacement combination valve appears to be a little bit hit and miss.
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Old 10-08-2015, 09:29 AM   #338
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Boy, from the looks of that schematic and considerring that there is mechanical/hydraulic/electrical components all involved in the activation of the brake light switch, and its a 35+ year old vehicle, I think I would be tempted to just deactivate the light and call it done. Doesnt seem that the brake light is really telling you anything more than what you would feel in the pedal. However I realize with the amount of time, money, planning, etc, that youve put into Bella that you want EVERYTHING to work as designed.

Do your brakes feel capable at this point? Kind of a hard question to answer unless you have another unit to compare it to. It took a good deal of work getting mine to a point where I feel very confident in them but Ive never driven a vehicle of that size before so getting them "right" is kind of subjective. It seems to me that the p30 series brake design in general is a challenge to deal with. I had two large truck mechanics tell me that my brakes were working as designed and I just needed to get use to them.....NOT.. New pads, flex lines, master cylinder, multiple bleedings, complete fluid flush and finally adjustment to the pedal connecting rod later.......NOW they feel like they will stop the vehicle with confidence.

Keep the faith , the Airstream Gods owe you one (or 10) at this point, with all the love youve put into Bella.


Mike
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Old 10-08-2015, 12:04 PM   #339
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All good points but here is my concern Mike.

The brakes aren't worse when the light comes on but aren't good anyway. Like you, the guys at Airstream said the brakes are working to spec (and I had them run the hydro-boost test in the Light Truck Manual), and I just needed to get used to it. But (and this is now totally theoretical) the switch is telling me there is a pressure differential (which could be proportioning valve or leak/air in one system) then it could explain the long pedal travel. The MC is designed with two chambers so if there is a issue (even a split hose) on one circuit, minimal pressure is generated on that circuit, yet the other circuit works. I am guessing one possible cause of my 'long pedal' is actually a problem that the warning light is sometimes picking up. Especially as the light resets when it is pushed back ie is reset by gaining pressure again.
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Old 10-08-2015, 12:11 PM   #340
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In my opinion the brakes on these coaches are problematic at best. On the 310 that I used to have when I first got it I took it to a large truck shop to have the master cylinder replaced along with hoses and a front brake job.

After a week or so they called to say it was ready so I picked it up, drove out of their lot and almost couldn't stop coming to the first stop sign I came to as the pedal went to the floor

Had to turn around and take it back and they ended up keeping for a month trying to get the brakes to work. In the end after they replaced all the components they then had to pressure bleed the brake system. Once they did that the brakes were "ok". Not great but ok.

I honestly can't believe that when these vehicles were new the brakes were as iffy as they are. Martin, If I were you I'd keep after them until you get a pedal feel that seems right.

Good luck!

Brad
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Old 10-08-2015, 08:21 PM   #341
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Martin , are you saying that the brakes do function consistently and they feel about the same whether or not the light is on? Have you confirmed the parking brake switch is operating properly? I believe the parking brake will illuminate the same light.
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Old 10-09-2015, 02:46 AM   #342
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Yes the brakes feel the same....but the same bad, not the same good. But the emergency brake light is working fine.
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Old 10-09-2015, 06:00 AM   #343
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Yes the brakes feel the same....but the same bad, not the same good. But the emergency brake light is working fine.
You mention a "long" pedal. I always test for fading of the master cylinder, by putting pressure on the pedal. If it continues to fade down (without pumping), you have a bad Master Cylinder (or of course a leak somewhere). If you have to pump the pedal to gain heights and a hard pedal, you have air in the lines. Both of those conditions could activate the brake light, but also should have been detected by a brake service shop.
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Old 10-09-2015, 07:32 AM   #344
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The feel of the pedal is more that I push and feel good resistance (no fade), but it is only right at the bottom of travel that the pedal really stiffens up and the brakes work. The only way I can describe it is like the hydraulics are having to move the pads a long way before braking starts
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Old 10-09-2015, 01:09 PM   #345
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The feel of the pedal is more that I push and feel good resistance (no fade), but it is only right at the bottom of travel that the pedal really stiffens up and the brakes work. The only way I can describe it is like the hydraulics are having to move the pads a long way before braking starts
I once went through 3 Master Cylinders to get rid of the issue you describe, its kinda spongy before it starts to get firm.
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Old 10-10-2015, 01:39 AM   #346
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I will have the shop replace the MC first Peter.

It appears they tried to contact me late yesterday, but technology is letting me down a little as they left a voicemail, but I have come over to the UK for a couple of days and my service provider won't let me pick it up from here. Shop is shut until Monday so I will have to wait until next week to find out what they had to say.
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Old 10-10-2015, 07:48 AM   #347
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Martin

IMHO the only way you will get reliable braking ALL the time is to install one of these. If I was driving back in the old country I would seriously look at this.

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Old 10-10-2015, 08:53 AM   #348
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I had assumed these were supplementary systems. Are they intended as complete replacements?
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Old 10-10-2015, 11:17 AM   #349
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It's a supplement to your factory brake system. They really are pretty amazing. Martin check on them while you are in Europe, they are quite popular over there.

No I haven't installed mine yet. It's been a pretty busy summer with several short camping trips and now I'm in the middle of a boat building project. Maybe next year.
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Old 10-11-2015, 05:06 AM   #350
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Seems like I have some investigating to do, but first job is to get the brakes I have working as they should.
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