Question
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Followup To
Question -
Hello again Stephen,
Since writing about the brakes on this car a few days ago, I have made some progress in determining the cause of the brake pedal having so much travel. I got all the air out of the system that was possible using a power bleeder but still found the pedal traveling a lot more than normal...
Thanks again,
Dave
Answer -
I still wonder why it took so long to bleed the system. Did you use the proper sequence of right rear then left rear then right front then left front.
As for the pedal rod adjustment. The master cylinder piston must come all the way back, so, the push rod is adjusted to just elimanate free play when the piston is fully returned.
As for the long pedal after eliminating all air. All automotive hydraulic brake systems are supposed to retain a residual pressure in the lines--10psi for drum brakes and 2psi for disc brakes. This is controlled by a "residual pressure" valve located in or near the master cylinder. Yours are missing or defective. And, yes, I have diagnosed this previously when someone replaced drum brakes with disc and retained the original master cylinder. If need be you can get these valves aftermarket at Summit Racing.
Did you get my email about the rear pressure regulating valve...
Hello and thank you again for your replies. I now realize that some of the things I did on this brake job - and been doing for many years - are wrong.
Usually, when I bleed brakes, I just open the caliper/wheel cylinder bleed valves, one at a time, (in order of farthest from the M/C to closest) and let gravity bleed them. That usually does the job. However, if it doesn't, I will try to get someone to pump the brake pedal up, hold it, and then bleed them under moderate braking pressure. (I now understand that if the brake is pumped quickly, that can entrain air in the fluid and cause problems. If it is pumped past the point where it normally stops, that can push the seal past a rough edge in the cylinder and ruin it - which I think might have happened in this case.) If that doesn't work, I hook up my pressure bleeder. That's what I did on this job - with what now appears to be a bad M/C, which is why I ended up going through half a gallon of fluid, trying unsuccessfully to get things right.
Regarding pedal adjustment, the push rod - and therefore the piston - does return the full distance back in the M/C. The push rod adjustment affects only the stop point of the pedal. There is no other means of stopping the brake pedal or adjusting its rest point in this car. I had a co-worker slowly press the brake pedal while I was under the car to see what was happening and perhaps determine if there was something else (wear, etc) causing such travel. The issue is definitely that the piston in the isolated M/C travels an unusually long distance down the cylinder before being stopped by compressed fluid. I have heard that a bad piston seal could be the cause of this but still can't understand why fluid wouldn't then be leaking. There are no leaks anywhere. There's only one piston seal. If fluid gets past it, it should escape into the dust boot or otherwise appear outside the M/C. But it doesn't. This is where the mystery is. Fluid is not back flowing into the reservoir. Nevertheless, it is going somewhere. The brake pedal travels easily and stops hard. No sponginess. I have bled it like crazy and it doesn't feel like there is any air in the system.
The 'residual pressure valve' you referred to is not on this car. The only other seal internal to the M/C is the one that prevents the backflow of fluid into the reservoir. When I rebuilt the M/C, and during the 'geyser' test of the reservoir, I believe I established that this valve was working. However, I went to Summit Racing's website and saw the part to which you are referring. This car certainly qualifies as one that could potentially need it. But I'm curious as to why it wasn't used in the first place. Alfa engineers certainly knew what they were doing...
I saw your reply concerning the rear pressure regulating valve, but only after writing the previous follow-up. Sorry about missing that earlier. Regardless, the car doesn't have one. Nor does it appear in the factory parts manual. Nevertheless, as a result of isolating and testing it, I'm pretty certain that the M/C is the problem although why is still unknown. I have ordered a replacement.
Thank you again for taking so much time with this. Your knowledge is impressive and I appreciate being able to discuss this with you.
Sincerely,
Dave
AnswerLet me clarify. There is a hole in the bottom of the reservoir that functions only when the piston returns to full rest. It allows fluid in both directions. The residual pressure valve is a spring arrangement on the end of the piston in this case. There has to be a pressure regulator for the rear brakes, either it was removed or you just missed it--it can be located anywhere on the brakeline to the rear. Finally, you very well may have a defective m/c...
