Brakes lack stopping power at times

[Graydon]

My brakes are lacking stopping power at times. It seems to happen more often when the AC is on. The pedal is firm and I've checked the pads and shoes and they are good. I bled the brakes and didn't see any signs of a leak anywhere. The brake booster tests OK and there isn't any hissing sound from the dash area. I still need to test the check valve for the booster, could the check valve be the cause?
Thank You
1989 Full camper, stock 2.1 with a manual transmission.

[PDXWesty]

I've experienced the same feeling on my van at times and my system is functioning properly. When I'm fully loaded, the van seems to take a while to stop if I slam the brakes from any kind of speed over 30 mph. I just think the brakes are undersized when you get up to about 5000 lb vehicle weight. That's why so many vendors sell bigger brake kits.
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89 Westy 2.1 Auto

[Ahwahnee]

Do you still have stock wheel/tire size or are you using something larger?

[Graydon]

This just started, until now the brakes were OK. I've had the same size wheels for sometime and never experienced a problem. I have the Gowesty 16" wheels with 215/60 16 tires.
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1989 Westfalia full camper 2WD, 4 speed with 2.1 WBX.

[Graydon]

Tested the check valve today and it tested OK. The lack of normal stopping power only happens at times which makes it difficult to trouble shoot.
Could it be the booster or proportioning valve starting to fail? I've read this rarely fail.
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1989 Westfalia full camper 2WD, 4 speed with 2.1 WBX.

[Abscate]

Could be internal corrosion , especially if you haven’t replaced brake fluid every 2 years
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[IdahoDoug]

Your AC being on may mean the throttle is kicked up slightly which in turn reduces the creation of vacuum to keep refilling the vacuum booster. So yes it could be the slight indicator your booster is on the way out. Easy test. Or it could be a leak in the looooong vacuum line from the engine to the dash mounted booster.
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1987 2WD Wolfsburg Vanagon Weekender "Mango", two fully locked 80 Series LandCruisers. 2017 Subaru Outback boxer. 1990 Audi 90 Quattro 20V with rear locking differential, 1990 burgundy parts Vanagon. 1984 Porsche 944, 1993 Toyota LandCruiser, 2002 BMW 325iX, 1982 Toyota Sunrader

[Graydon]

@ IdahoDoug Thanks for the info on the idle. I did replace the idle control stabilizer unit recently. It wasn't kicking up the idle with the AC on. Now it is idling a little high. I also adjusted the throttle position switch. I'll check the basic setting for the idle adjusting screw today.
With the exception of the proportioning valve, hard lines and the booster the brake system was replaced 3 years ago.
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1989 Westfalia full camper 2WD, 4 speed with 2.1 WBX.

[Mellow Yellow 74]

Try manually adjusting the rear brakes - I never did because they are supposed to adjust automatically but they were losing performance so I adjusted them manually and it made a huge difference.
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1962 Karmann Ghia
1974 Deluxe Microbus
1985 Caravelle (Vanagon)

[dhaavers]

x2

When van was new to us, first job I did was rear brakes. Even then, I'd gain some pedal
travel & lose some stopping power every couple years. Adjusting rears always brought
things back to perfect. I've started taking the time to adjust rears at the start of each
driving season (van is garaged Nov-March) with no brake surprises since...

- Dave
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86 White Wolfsburg Westy Weekender
"The WonderVan"
<EDITED TO PROTECT INNOCENT PIXELS>

[Graydon]

Problem solved..
The rear brakes needed adjusting. Everything seems to be back to normal now.
Engine still idles a bit high @ 940 rpm, I'll adjust it before we head up to Oregon for Descend on Bend.
Thank you to all for your advice.
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1989 Westfalia full camper 2WD, 4 speed with 2.1 WBX.

[dhaavers]

Have a great trip!

- Dave
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86 White Wolfsburg Westy Weekender
"The WonderVan"
<EDITED TO PROTECT INNOCENT PIXELS>