308 brake peddle goes 50% to floor only when motor is started

It could be several things. air could be in the system or the master cylinder internal seals are on their way out, and perhaps the cylinder itself depending on the condition of the walls which sometimes pit. The flexible brake hoses themselves also like to flex when depressing the pedal if they are on their way out.

 

Pedal.

 

I drove my '81 for two years without any check valve at all (so had to be a bit careful not to stall the engine when braking!). I had the same behavior you describe, where the pedal would drop as the engine fired and built vacuum. Once there was vacuum I was back to getting power assist and of course the assisted pedal feels a bit softer than the unassisted pedal (which feels pretty much like stepping on a brick).

One interesting thing: the motor would speed up a bit when I first touched the brake pedal, then settle down again. I believe that was caused by a momentary change in manifold vacuum as the booster charged.

 

When the engine fires up, it pulls on the diaphragm, applying additional pressure. If the pedal goes down, there must be something getting tightened up, or the master leaking fluid, or air in the lines.

A couple of things that might cause additional travel are loose wheel bearings, allowing the disc to tilt a bit, between the pads. more pressure could cause the pads to tighten up on the disc, cocking the pads in the caliper bores a bit - not a good thing.

The rear parking brake mechanism could be sticking or out of adjustment, requiring more pedal travel to make solid contact with the disc.

Hope this is of some help,
chris

 

My experience is that most cars with brake boosters will feel different between not running and running. How do your brakes operate? Do they work well?

 

Stuck open is the same as not having a check valve at all; no power assist when the motor isn't running. Stuck closed would mean no power assist ever.

It's easy to test the check valve. Just remove it and try blowing through it in both directions.

But I have to ask: are you still seeing the brake warning light come on when you brake? That has nothing to do with check valve and tells you that something somewhere in the brake hydraulics is NOT in good condition.

 

I had a similar problem and it was the check valve. I had erratic vacuum with the brakes and soon as I changed the check valve also known as a non return valve it solved the issue.

Not sure what part number it is on your 308 but on my 3.2 Mondial which is the same as the 328 it is no longer available in its original format and has been upgraded and therefore a new part number.

On mine it's the same as that used on the 360 and 430 etc. The original part was metal and the replacement is plastic.

The good news is that it takes 2 minutes to swap over and cost me about $8 USD. It's worth a try before spending big bucks

 

It is not unusual for the brake pedal to go further towards the floor once the engine is started on a car with power assist. As mentioned, this is because once the engine is started additional vacuum is applied to the booster unit. This may occur because even with a working check valve once the engine is shut off the vacuum will leak. Additionally, without the engine running each time you press the brake pedal and release it some vacuum is released.

You may also notice that while driving the car, if you take you foot off the throttle and depress the clutch, allowing the car to coast with the engine at idle, if you repeatedly press and release the brake pedal after a few time you will have to apply greater foot pressure to stop the car. This is because either the check valve is leaking or stuck open, or more typically, the engine can not provide sufficient vacuum to the booster at idle. As long as the car stops properly with the engine running at greater than idle there most likely isn't a problem.

You can verify that the check valve is working by running the engine with the car parked. Rev it up a little (3k RPM will to) for a second or two ( a couple of throttle blips). Then shut it off without applying the brakes. Immediately go to the engine bay and disconnect the vacuum hose to the booster up stream of the check valve. When the hose is pulled off you should hear a pop if there is good vacuum. This indicates that the check valve is holding the vacuum.