BC

https://catuned.com/chase-bays-brake...002-porsche-vw

This specifically says it fits a 928.

BC

I guess maybe not many these days

Rob Edwards

Didn't realize such a thing existed for 928. Wonder what the pedal effort would be like?

BC

That is a concern.

Really having an issue with this master replacement and bias valve install for the 79. Completely flat pedal when car is running.

Mark Anderson

Joseph tried this on his race car. He spent $$$$$$$ and gave up

Imo000

What's a flat pedal?

BC

Wish I could hear the story on that.

BC

I either have air trapped because of the different plumbing for the boas valve on this early car or I actually have a bad (new old stock) master. The pedal Goes to the floor with the booster under vac. Engine off, pedal is very soft.

Im going to bench bleed and then test this master.

Mark Anderson

Pm him. Poc62?

BC

thanks.

BC

I just did, thanks.

He was last active in August though, so...

The Forgotten On

Honestly sounds like air is trapped in the system or the seal plugs that attach the reservoir to the master have failed and are allowing air to get past them.

Bleed the system again and see if it makes a difference.

Personally, I would be weary of a NOS part like this as the rubber ages regardless if it is installed or not. Also, the bores could have rusted from humidity and mangled the seals if stored in less than ideal conditions.

It's just me guessing at this point as I haven't seen your car and its exact symptoms.

The bias valves under the master are pretty robust in my experience though and shouldn't cause any issues unless the car was bled improperly.

Just check that they are installed correctly and that the arrows on them showing the flow of fluid are in the correct direction.

BC

I have caused my own problems here. As usual.

I removed the bias valves. I have placed a T in AN pieces below a Tilton proportion valve coming out of the front of an S4 master for the rear brakes. Fronts get all copper line into an oem Junction.

I have bled two quarts through. No more bubbles, pure clean fluid out of all
bleed orifices.

I am using motive brake bleeder. Over 10psi of pressure.

I am am going to bench bleed and test some masters I have today including the one on the car and see what I see.

V2Rocket

What is the bore of the 928 mc?
Luckily wilwood can make these mcs in any size you want. Smaller bore=easier pedal effort but longer pedal travel.

I've driven a 944 turbo with depowered, stock 15/16" MC - it stopped, but you had to want it to stop... rock hard pedal requiring very high pedal effort. Id probably go down to 13/16 or 3/4 were that my car.

if accessible pull the vacuum line off your booster and plug the manifold side, booster can stay open for the test. Try driving your car down the street you live on, be prepared to use all leg strength to stop.

dr bob

BC --

The early cars have diagonal braking, meaning no (single) bias valve is possible. Are you changing to later MC and plumbing?

The early diagonal-braking MC's MUST get a full bench bleed before installation. There are two piston sections inside that are not mechanically connected unless there's a failure of the rear-piston hydraulics. The rest of the time it's hydraulic pressure from the rear piston that pushes on the front piston. The system keeps the pressures from the pedal balanced between the two circuits. On either circuit hydraulic failure, the pedal will go pretty low before brakes are applied on the surviving circuit. The warning light sets when there's more pressure on one side than the other.

The pedal-bleeding procedure for diagonal-braking cars is usually a bit different from what we learned on conventional systems. I think the WSM has the details. Meanwhile, low-pressure bleeding with the power bleeder is less particular, but the system still "likes" bleeding the circuits in the "correct" order.