1980 Malibu master cylinder- is it working correct?

[Caprimaniac]

Hello, folks. I installed a 7" brake booster in a Europe- car as there was no room for a stock one.
On that booster, I installed a Chevy Malibu (78-80) master cylinder. 0.750- bore. Got a middle-of the road priced master at around 60$. "Raybestos PG Plus Professional Grade Brake".

After installing hard lines connected to PTFE hoses, it was time to fill the system. However- I couldn't get a drop of brake fluid to escape through the lines. It took 3 days to complete, but I got a hard pedal in the end.

A month ago I had to change brake- calipers. OK. Again- had to bleed the system. Same problem; fluid won't drain into the pipes. Got a vacuum- bleeder. It did work; fluid was drawn in. I have a pedal- but not sure if it's good enuff.

Why won't the fluid, by gravity, fill the lines?

I also tried filling the system by pushing the pedal with open vent- screws and a hose into fluid- filled bottle without success. Vacuum was the only way...
Another symptom while pressing the pedal was: Air or fluid (hard to tell), squirts out the rear inlet into the reservoir (and spilling fluid in engine compartment).

Also, another "problem": When I press the pedal, there is fluid coming out the front inlet into the reservoir.; not a fountain, just a calm ouze... . Is this normal?

It's plumbed front chamber- prop. valve single line rearward splitting into two. Rear chamber goes to a T and splits into front R and L.

Really tired of this mess; taken many weeks to get brakes working- and not finished yet.
I did take the master apart & inspected it. Nice shine to the bore; perfect. All rubbers like new. The master has been on the car for a year; bought new ofcourse.

If you have any input.... Please come forward!!!
Thanks

[FC-Pilot]

This does not surprise me. I use to work for a vehicle testing company and about 85% of what I did was brakes. I have seen some of what you are speaking with older style stuff. Most of the modern systems I have worked with have not been as big of a pain. Now, we also used pressure blenders that provided pressure from the top of the master cylinder with a constant flow of fresh fluid. This made the basic process of bleeding easy. Now, what we found was that still did not get all the air out of the system. One area we found over about a year of experimenting with for Brembo was the calipers ALWAYS held some bubbles around the pistons and seals in their bores. We could always improve (shorten) the pedal travel by 2-3 mm (It does not sound like much, but in our world it was vital) by bleeding the system, then compressing the pistons back in the bores, while tapping with a rubber mallet on the caliper. Also found that about five or more quick smashes of the pedal while blending would often break a bubble or two lose in the lines. Lastly, if ABS equipped, doing full ABS stoops in dirt of loose gravel would offer. Help free up those last bubbles. I know it sounds a little archaic, but it got results.

I then got hired by Toyota and when I explained this stuff to the engineers they told me I was wrong and was an idiot. I them broth them data they could not believe. When they asked how I produced those numbers I showed them the procedure and verified it with data logger and instrumentation setup as standard (to show I was not “fudging” the numbers) which upset them even more. They did not like being wrong.

Long story short, keep working at it. Also, your master cylinder bore to piston bore diameter is very important. If you get it out of wack that could easily produce a poor feeing pedal.

Paul

[BillK]

Make sure that the pushrod is allowing the master to come all the way back when the pedal is released. A quick test to see if this is the issue is to loosen the master cylinder mounting bolts a few turns and see if anything changes.

[dannobee]

The fluid oozing up is normal. The compensating port is still open at the beginning of pedal travel, so the fluid goes back into the reservoir before the piston moves far enough past the compensating port and pushes the fluid out of the outlet.

The bleeding problems sound like a combination valve issue. Was the brake warning light on when this was happening?

I'm with Bill. If the master cylinder rod is being held down, the only way to get fluid to pass is with vacuum, as the seals will only hold in one direction.

[lefty o]

worst case if you cant get this sorted out, grab a 9" dual diaphram unit out of an 87/88 monte carlo along with matching master cyl.

[Caprimaniac]

Thank you all.

No room for a 9", Lefty.

Confident that the master cylinder rod is free'd up. I have no valves as for waning light or other in the curcuit.

Will keep working on it. As a matter of fact; I would like to change all brakelines as I can borrow a proper tool for making the flanges MUCH better than with my handheld.... Also got burned by a set of china- calipers where the handbrake- pivot inside the calipers did not return.

Good to know pressure/ vacuum- filling the system is "normal".

Thanks again