Power brake question

[mytrplseven]

My turbo has the brakes rebuilt, new lines and new master cylinder. I've flushed the system and bled it numerous times but even when I stand on it, the brakes won't lock up. It will stop after it slows down but I'm not sure if it's the booster or not. I'm asking the forum if they can tell me how much the brake pedal goes down when you start the engine? Mine doesn't seem to move noticeability when the engine is started. I've put 10inches of vacuum on the booster line and it holds it until I push the brake pedal which I determined as normal. I did install the Lindsey Racing venturi elimination hose package and I'm generating about 12 inches of vacuum on the booster line.

[v2rocket_aka944]

Pedal doesn't move on 944 when engine starts.
That vacuum reading sounds low?

[djnolan]

First I would make sure the brakes are bled properly. Booster or not, if bled properly, you should be able to lock up all 4.

Longer story: The best way to bleed brakes was shown to me buy a guy who's father raced at Indianapolis. Two persons are required. One pumps and holds down the pedal to force the air to the end of the line. The second person cracks the bleeder screw to let out the air, and then closes it quickly. Once this is done a few times, the pedal will be rock hard. No flushing or Motive or other bleeder equipment is needed to achieve a rock hard pedal. Just get the air out by pressing down the pedal firmly and cracking the bleeder screw for a instant.

For some vehicles the engine needs to be running and for others not. It has something to do with the brake booster obviously.

[mytrplseven]

That's the procedure my friend and I performed and then went back to make sure no air got backed up into the lines. When I stand on the brakes, the pedal is firm and the car slows dramatically to a full stop but I can't lock the wheels. I had heard that power brakes booster causes the brake pedal to move inward a small amount when the engine is started. V2rocket says it doesn't on the 944. I'm going to an autocross tomorrow and will talk to other racers about what they think after they drive my car.

[mytrplseven]

To V2Rocket: what kind of vacuum should I see from one of the vacuum taps on the turbo intake manifold? I can't seem to find anything written that defines it.

[v2rocket_aka944]

https://rennlist.com/forums/944-turbo-and-turbo-s-forum/64960-what-is-an-average-vacuum-reading-2.html

[mytrplseven]

Bought a quality vacuum gauge today and after hooking it up I had around 18 inches of vacuum at cold idle and 21 inches at cold 2000 rpm. I guess I'm in the market for a new brake booster. I really look forward to this project....not.:


IDLE



2000 RPM

[djnolan]

Originally Posted by djnolan

if bled properly, you should be able to lock up all 4.

Actually I have never locked up all 4 on a 944. I think with the 50/50 weight balance it may not be possible! Should we start a poll?

[mytrplseven]

You may have a point there. All I know is I had to virtually stand up on the brake pedal to get it to stop. With no booster, it's very noticeable. New booster coming in this week.

[Stoneguy]

Mytrplseven,
At the beginning of the summer I did, basically what you did on my brakes. I converted a standard 944 to the Brembo Porsche calipers and setup that is on the turbo. After the work, calipers, rotors, pads, master cylinder bias valve addition struts and spindles, I was a little disappointed with the feel. I replaced the booster. I had checked it out and it SEEMED ok, but a 34 year old booster it was. After the booster change, my brakes are wonderful. Just my experience.

[mytrplseven]

Thanks for the supportive comments. I'm right in the middle of removing everything under the hood so I can pull the booster out. On a 951, it's a complicated process in confined spaces.

[mytrplseven]

After a lot of work I finally have the booster installed, bled the brakes and did a road test. NO CHANGE. I still don't have boosted brakes!! When I start the engine, the pedal does droop a little like most other cars when they are started with foot on the brake. I simply do not have a clue where to go next. Another question about the Brembo calipers: Since they have two bleed nipples, is there a proper procedure for thoroughly bleeding the calipers? I've only been doing the outside bleeder nipples. The un-boosted brake pedal is solid, though and the car will stop by standing on the pedal, just not like a power brake system.

[kdjones2000]

Bleed them both.

[mytrplseven]

Is there a recommended order, i.e. inner-outer or outer-inner?

[jjeffries]

777, does anyone in your area have a similar car which you could compare (driving/braking sensation) to yours?

I suppose that begs the question, did the car always feel
like this or has it deteriorated over time?

John

[kdjones2000]

Originally Posted by mytrplseven

Is there a recommended order, i.e. inner-outer or outer-inner?

Not that important, although I would start inside first then outside. You need to bleed both halves of the calipers of all air. The fluid goes from one half to the other via that "U" tube at the bottom.

[mytrplseven]

Jeffries: Initially it worked fine but during the 5 year restoration process the brake system was completely overhauled with new lines, master cylinder, brake booster, check valve, rubber lines and rebuilt Brembo brake calipers, Zimmerman rotors and Hawk pads. Vacuum readings into the Booster were well within spec and the pads were not glazed. A Lindsey Racing venturi delete system was also installed.

[thomasryan]

bleed both sides but air in the system is a spongy pedal.

have you confirmed movement of all the caliper pistons?

does the master bore match the caliper upgrade(s)?

are you getting the pads up to temperature?

[mytrplseven]

Originally Posted by thomasryan

bleed both sides but air in the system is a spongy pedal.

have you confirmed movement of all the caliper pistons?

does the master bore match the caliper upgrade(s)?

are you getting the pads up to temperature?

I wouldn't say the pedal is spongy. When we manually bled the brakes the pedal was firm afterwards. The master cylinder was replaced new and the proper one for the car. I don't know what "upgrade" you're referring to as I just cleaned up the pucks and replaced the seals and cleaned the bores. I ensured the pistons and pads moved freely after rebuilding the calipers. The pads aren't competition variety so having them needing pre-heating isn't an issue. that said, I did ride the brakes and used them hard during the road test so I don't feel that's an issue. Also after a temperature test of the rear rotors, they were about 25% cooler than the fronts so the proportioning valve is doing it's thing.

[djnolan]

Definitely make sure both sides are bled completely. I had a spongy pedal problem once that turned out to be a clogged bleeder screw on the rear. There was soo much air build up from oxidation in the rear lines that the only braking that was occurring was in the front. The car was really diving when I hit the brakes.