240 - Brakes suddenly applied while driving!

[seidlitz]

If you can explain this problem, you are a god!

Recently, while driving, the brake pedal made a popping sound and actually rose maybe a half inch or an inch. At the same time, the brakes were applied - not all-out, slamming brakes, but as if I were pushing the pedal halfway down. If I pumped the brakes a few times, they would relax, but then 5-10 seconds later the problem would reoccur.

I took the car to the nearest workshop (quality or trustworthiness not guaranteed!), where they replaced the "brakes master pump" and the back brake pads. Cost me about $90 (I live in Malaysia). Within a day, the problem resurfaced, so I brought it back to the same place. They replaced another kind of pump (on the receipt it looks like "body pump" but I'm not sure). Cost me another $50.

It's been fine for 6 weeks but just happened again today. I can't, for the life of me, figure out why the brake pedal goes UP when the brakes are suddenly applied. The only thing I can think may be a factor is that I used to use a security lock which was like a hook, applied to the steering wheel and brake pedal rod. Maybe the pulling pressure on the lock, over time, did something?

Anyway, like I said, if you can explain this, I'll be forever in your debt and will proclaim your status as a car god to everyone I know.

[LaRy]

I have never heard of a problem like this, but theoretically it can happen if it is something wrong with the valve placed inside the brake servo canister.
Actually what you are doing normally when braking, you are closing a valve with the brake pedal. This valve is placed inside the canister. Then the vacuum will move a big piston inside forward to put pressure to the pistons inside the brake cylinder. If something is stocked inside this canister, this fault could be possible. You must definitely do something, I would change the brake servo canister or ask some specialist to have a look at it!

[Volvord]

A very interesting problem. You do not mention the year of your 240 or if it has ABS (antilock brakes). As LaRy said it could be the vacuum booster but this would be a very long shot, even with direst vacuum there is not enough force applied to the diaphram to apply the brakes without some input from the pedal ... but then again I have seen stranger things.

If your car has ABS I may suspect that a combination of an actuator valve and a retriction in a return circuit may be the problem. If you do not have ABS I would consider a mechanical problem rather than hydraulic, a loose caliper, wheel bearings, faulty caliper (sticky piston), emergency brake.

The parts you list are not common names for components in a braking system so I am not sure exactly what work was performed. Major components would include; master cylinder, brake booster, porpotioning valve, calipers, and if ABS equipped they have a non-servicable ABS unit which costs considerably more than any of you charges thus far.

[Guest]

It's a 1990 Volvo, and I'm not sure if it has ABS or not. Sorry, I'm a bit of an ignoramus when it comes to cars!