On a recent trip in British Columbia in our motorhome, our route included a lovely scenic highway from Lytton to Lilloet, and another from Lillooet to Pemberton. It was a beautiful drive, and very rugged terrain.
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The first stretch was steep and winding. The second stretch from Lilooet to Pemberton was even worse, with hairpin turns and very steep grades up to 13%. The 13% portion was close to Pemberton, and there were signs saying "Recommended truck speed 10 KM/h".
For a few minutes before the trouble started I noticed a slight burning smell.
We were descending, when suddenly on a hairpin turn my wife noticed a hubcap had come flying off.
You may recall I installed new hubcaps this year (described in this post). In order to retrieve that hubcap, I stopped at the next pullout, which turned out to be very short and very steep; thus I had to brake hard. We could see smoke coming from both front wheel wells, and the smell was unfamiliar to me. The other front hubcap was gone too, but the rear ones were fine.
After retrieving the hubcap and waiting for the smoke to stop, we started out again and I noticed almost immediately that the brakes were very spongy - I could almost push the pedal to the floor. I immediately geared down to the lowest gear, limiting our speed, and we crawled down the mountain as slowly as possible.
Once we got down, the terrain levelled out and the brakes seemed much better, so I decided to drive to Pemberton to seek help. Within 10 minutes everything had returned to normal.
I am not a mechanic and will ask for some confirmation when I have the brakes inspected, but from my research I am pretty confident this is what happened:
We had experienced temporary brake failure; sometimes called "overheating brakes", "brake fade", or "brake fluid vapour lock". The best description I could find can be found at this link.
Due to the heat of the day, the steep terrain, my heavy vehicle, and my haste to get to our destination, the brakes had overheated. This resulted in a small amount of the the brake fluid near the calipers coming to a boil and turning into bubbles of gas, and also forcing a little fluid out onto the hot brake parts resulting in the burning smell. Gas is highly compressible, where brake fluid is not; thus the brakes became spongy. Once I had driven at low speed for 10-15 minutes the brakes cooled somewhat, the gas condensed back into fluid and the brakes were fine.
This may have been exacerbated a bit by the fact that I was using hubcaps on those front wheels. The hubcaps could have retained some heat near the brakes. I will need to do some research before I commit to installing new front hubcaps. Very likely the heat from the brakes caused the hubcaps to become loose, either by weakening of some plastic behind the retainer clips or maybe a little expansion of the rim itself.
Next step is a trip to the mechanic for an inspection and a brake fluid flush and fill. The fluid becomes a little more prone to boiling over time and should be replaced regularly anyway.
I'll update this post later if I find out more!