mytrplseven

I have rebuilt the Brembo calipers, replaced all the brake lines, both hard and flexible, new master cylinder and most recently, the brake booster. Although the brake pedal droops slightly upon engine start (as it should), I still have no power brakes. Attached are pics of the readings of my vacuum source going into the booster, which by the way, has new hoses and check valve replaced with new. I now have a question about bleeding the Brembo brake calipers, although I don't feel they are at the root of my issues. Since there are two bleed nipples on each caliper, is there a recommended procedure for fully bleeding each caliper?



vacuum at idle


vacuum at 2000 RPM

MAGK944

So you have only confirmed the vacuum going to the booster, correct?

Do the brakes perform the same with or without the booster connected to vacuum?

Have you confirmed that the new booster check valve is working correctly by blowing and sucking on it when it’s not on the booster?

With the Brembo brakes you bleed each side of the calipers separately, so you bleed each caliper twice, inner and outer.

Edit: Just re-read your post and the brake pedal does not droop slightly upon engine start on a 944, so you may have a booster/vacuum issue as well as a brake bleed issue if you didn’t bleed both bleed screws in each caliper.

jerome951

I bleed the outer bleed screw first, then inner.
This way, if any bubbles in the line enter the caliper (they enter on the inboard side), they will float up and be expelled when I bleed the inner screw last.

To clarify, though, do you have no power brakes, or no brakes at all (pedal goes to the floor)? If the former, bleeding likely isn't the issue.

mytrplseven

I have brakes but it's like the power brakes portion of the system has failed. I really have to stand in the pedal to make it stop. Tomorrow I'll put her up on stands again and do a thorough caliper inner and outer bleed and then there shouldn't (hopefully) be that part of the system as suspect.

mytrplseven

I did check the check valve that way and was good. Brakes perform like there's no power booster.

mytrplseven

Following a suggestion from a familiar forum member, I pulled the vacuum line that was connected indirectly to the vacuum reservoir. Sucking on the line showed it wouldn't hold vacuum. It wasn't connected at the reservoir behind the battery. it seems the PO had broken it off and did a marginal job of gluing it back on. Replacement on order.

Droops83

Are you referring to the small vacuum reservoir in the cowl area that holds vacuum for the HVAC system? If so, that does not have much effect on the brake booster. The booster has a large, dedicated line to the intake manifold---even if another small vacuum hose is disconnected from the manifold, that small of a vacuum leak will not cause a total loss of vacuum to the booster. If you had a bad enough vacuum leak to cause that, then the engine would barely run if at all. Also, your vacuum gauge readings in the first post seem to indicate that you have adequate vacuum at the booster.

It might be time to get the car to a shop that has a good set of brake hydraulic pressure gauges and see if your issue is there.

mytrplseven

I was wondering if this was going to be the root of the problem or not. As I move through all the systems, I find things that need to be replaced anyway but with the vacuum shown and a new booster and master, plus rebuilt calipers, new pads and rotors and numerous bleed sessions I'm running out of ideas. While I appreciate the validity of your observations, I'm curious about what hydraulic testing you were referring to. Could you elaborate further?

mytrplseven

Replace the vacuum reservoir and as you suggested, it didn't have any effect on the overall vacuum power brake system. So now I'm on to trying to figure why the power brakes weren't working.

Droops83

Are you sure that all 4 of your brakes are engaging? Your signature says '86 951 and I assume this is the car you are working on, and it doesn't have ABS?

There is a proportioning valve that is screwed into the rear brake circuit outlet port, at the front of the master cylinder. It is possible that this is seized internally and you are only getting front brake activation. This is why it might be useful to find a shop that has a set of brake hydraulic pressure gauges that thread into the bleed screws. This would determine if you have an actual brake hydraulic issue.

mytrplseven

After doing an aggressive road test, I checked all the brake rotors with a lazer temperature gauge. The fronts were both the same temp and the rears were about 20% cooler and the same both sides. In summation, Here's the list: I've replaced the rotors, pads, booster, check valve, master cylinder, vacuum reservoir, booster input hoses, vacuum hoses throughout, installed the Lindsey venturi delete kit and rebuilt the calipers. Brake booster line vacuum is 21 inches hg at 2000 RPM. At idle it's 18 inches hg. STILL NO POWER BRAKES. I simply have run out of ideas.

Mike Goebel

Just went into the garage and checked if my pedal goes down on start up, yes it does. With engine off I pushed on the pedal and it went a small distance and then stopped and the pedal feel is very frim. With foot on pedal started the engine and the pedal went another 1/2-3/4 in further.

Mike G.

P.S. my brakes work bitchen!

MAGK944

After it’s sat for a while yes, all it means is the vacuum on one side of your booster has leaked while it’s sat and starting the engine momentarily replenishes the vacuum and moves the booster diaphragm causing the atmospheric (pedal) side to get sucked in/down.

Turn the engine off after a trip and then immediately see if it does it when you start it up, it shouldn’t on a 944 as we have a dedicated check valve. A lot of modern cars don’t and they leak out vacuum from the booster as soon as the engine is off. So they have a considerable pedal drop when the engine starts whether they sit or not.

PorscheFanatic202

What brand rotors and pads did you go with? I am having the same issue as you. Brakes were super touchy before my 3 year engine rebuild. After, they are nothing special and take a lot of force from me to stop it. I did notice that during power bleeding, I would hardly get a trickle of brake fluid at 15 psi, so I changed the flex lines. The fluid flow was much better during bleeding and braking is a little better, however when doing hard stops it is hardly impressive and takes a lot of force to stop it. I understand your frustration.

Humboldtgrin

Did you bench bleed the master cylinder before installing it? I would think you did. Are any of your brake hard lines smashed (like an off the track experience that could have smashed the rear brake hard line. And try bleeding the inner caliper pistons before the outer, why? It just makes me “feel” better. It sounds like your rear brakes aren’t working to me. You may need new brake hoses, or proportioning valve like stated before, they do eventually fail and will cause problems just as you describe. If the rear brakes aren’t working you can dam near stand on the brake pedal and cruise to a stop in middle of an intersection, even with good front brakes and operational booster. I know this first hand.