Losing the brake pedal...What the...?

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Jivermo
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Losing the brake pedal...What the...?

Post by Jivermo » Tue Jan 28, 2014 8:42 am

I had an interesting experience last week. I was going on a short run in my '78 Westy, and as I approached a red light, I could feel the bus pull slightly to the right as I applied the brakes. Also, I lost pedal on the brakes. I pumped them and they took hold, near the floor. I pulled over and shut down the bus, and checked the fluid reservoir, which was full. Strangely enough, the pedal returned to normal as I drove back home. I pulled all 4 wheels, and found no sign of leakage, or excessive wear. I've driven about 25 miles since then, and the pedal loss has not occurred again. However, it was quite disconcerting when it happened, and I would like to know if I should be looking at anything else, such as master cylinder or vacuum booster.

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Amskeptic
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Re: Losing the brake pedal...What the...?

Post by Amskeptic » Tue Jan 28, 2014 10:41 pm

Jivermo wrote:I had an interesting experience last week. I was going on a short run in my '78 Westy, and as I approached a red light, I could feel the bus pull slightly to the right as I applied the brakes. Also, I lost pedal on the brakes. I pumped them and they took hold, near the floor. I pulled over and shut down the bus, and checked the fluid reservoir, which was full. Strangely enough, the pedal returned to normal as I drove back home. I pulled all 4 wheels, and found no sign of leakage, or excessive wear. I've driven about 25 miles since then, and the pedal loss has not occurred again. However, it was quite disconcerting when it happened, and I would like to know if I should be looking at anything else, such as master cylinder or vacuum booster.
With engine off, parked, apply brakes firmly. Now release pressure without moving the pedal. Reapply pressure. Does pedal sink?

Now do the same, but this time, allow the pedal to almost imperceptably go up just a half millimeter and reapply the pressure, hold, repeat. Do you get any evidence of the pedal dropping down incrementially? Keep trying to feather the pedal down with intervals of pressure and no pressure.
If yes, master cylinder seals are not quite.
The cups seal under pressure. If the cup seals fail to develop pressure, we don't get the pressure to seal which doesn't let us get pressure enough to seal to get the pressure. Follow this tautology until you are dizzy. You never knew how amazing this process was . . . :geek:
Colin

"Rebuilding Your Leaky Cheap Master Cylinder Behind A Closed Belks Store"
viewtopic.php?f=33&t=11092#p194896
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

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