The standard test for the booster is to depress the brake pedal and start the engine. Pedal should drop a bit on startup.  Like Dennis said, you might have a stuck brake caliper piston. The friction could cause the fluid to boil (or absorbed water to vaporize) which results in loss of braking. Time to have the brakes checked. 

Stuart

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of Ryan Perry
Sent: Monday, September 25, 2017 5:55 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: Stumble and Loss of Brake Pressure Heading Up a Pass

Thanks Dennis, checked the level and was at max. I can't say how old the brake fluid is so maybe it's time to change it?

Is there a definitive test anyone knows for master cylinder and brake booster?
Thanks
Ryan

On Mon, Sep 25, 2017, 3:27 PM Dennis Haynes <d23haynes57@hotmail.com> wrote:

> Check master cylinder fluid level. Combination bad cylinder and brake 
> booster can let engine vacuum suck the fluid out of the master.
> Next would be brakes dragging or otherwise overheated and you vapor 
> locked the brakes. Problem could be compounded by the fluid being old 
> and saturated with water.
>
> Dennis
>
>
> -----Original Message-----
> From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On 
> Behalf Of Ryan Perry
> Sent: Monday, September 25, 2017 3:14 PM
> To: vanagon@GERRY.VANAGON.COM
> Subject: Stumble and Loss of Brake Pressure Heading Up a Pass
>
> This one is new for me, taking it to the collective brain trust!
>
> '84 Manual with a newly installed 2.2l GoWesty. Runs fine in all 
> normal driving conditions. When heading up a pass for camping this 
> weekend, think Western NC, couple thousand feet elevation over 4 miles 
> or so.  The van was under moderately heavy load in 3rd gear, RPMs on 
> the high side but nothing outrageous, was holding steady in power.
>
> About 4 mins in the power hiccups slightly then continues on. I had a 
> bit of a corner so tapped the brakes lightly and find no pressure. 
> Really weird.  Made it to the top for a short flat and found I had to 
> pump brakes to get pressure.  This was short lived for a single 
> application. As soon as I let off back to no pressure, had to pump 
> again. Then back up the next section of the cut under similar load and 
> had the same stumble at about 3-4min in then carries on fine. Top of 
> that cut and the end of our trip out the van is running fine, though a 
> bit hot as expected, but no brakes without pumping.
>
> Parked,  camped,  had a great time celebrating or 4th wedding anniversary.
>  (Someday I'll show you all or vanagon wedding pics, you'll like them) 
> Anyway, leave Sunday and brakes are fine, engine is fine.
>
> What do you think? Related? Coincidence? Vacuum line or booster? Fuel 
> pressure or pump?
>
> Thanks for any ideas, i try to keep stopping at the top of my list of 
> fixes!
>
> Ryan
> Asheville,  NC
>