Bench bleeding brake master cylinder issues

[laforzaland]

I am replacing the brake master cylinder on my 1980 Fiat 2000 Spider (FI). I purchased a new brake master cylinder, removed and thoroughly cleaned the brake fluid reservoir, and replaced the feed hoses from the reservoir to the master cylinder.

I have mounted the master cylinder in a bench vice (secured to the master cylinder mounting plate, not hydraulic tube-housing). I have the reservoir connected to the master cylinder via the new hoses and it's positioned at about the same height and position as it would be if in the vehicle. There are no kinks in any of the feed hoses. I have clear fluid return hoses attached to the two rear (output) ports and the one front (output) port that all terminate in the reservoir.

I pour brake fluid into the reservoir and it fills both sections. The fluid flows into the master cylinder and some exits slightly into the output ports without pumping. When I actuate (press the plunger) of the master cylinder fluid pressure pushes the brake fluid through both of the ports the feed the FRONT brakes easily, however NO pressure is evident at the single output port of the master cylinder that feeds the REAR brakes. I pumped the master cylinder for at least 10 minutes and still no pressure. I understand that the rear brake section of the master cylinder has a shorter throw as compared to the throw of the front brakes section, but there should still be pressure present.

I verified that the hoses were not clogged. I even purchased a second new master cylinder and the EXACT SAME THING is happening where there is NO pressure at the single output port that feeds the rear brakes.

Any ideas what I could be doing wrong?

[DieselSpider]

I did not bother with the bench bleeding since I went to using a MitiVac Vacuum bleeder however you could also vacuum bleed the master cylinder on the bench before you install it on the car. Best $35 I ever spent when a brake pad came unbonded taking out a rear caliper on a 35 foot motor home up in the hill country of Alabama and I had to replace a set of calipers by myself while out on an abandoned air field. Bleeding brakes by yourself can be a tough job when there is over 30 feet of brake line between the caliper and the master cylinder. Now that I have a Vacuum Bleeder I don't really know how I did without one all those years.

[klweimer]

If you have one of those small hand vacuum pumps, you might see if you can suck brake fluid through the tube you have attached to the rear outlet port. You could also try holding the master cylinder nose up or nose down in the vice to see if you can work out an air bubble. If you have a syringe, you could take the inlet side fittings off and squirt some brake fluid directly into the hole of the master cylinder. Do you still have your old master cylinder? You can see if it is pumping out of both ports. It should if your set-up is correct.

[laforzaland]

I have the solution to this problem (no pressure at rear brake port on master cylinder when bench bleeding).

How the FIAT master cylinder is built
When the master cylinder is actuated by the push-rod control arm in the power brake servo it mechanically moves the hydraulic piston that produces pressure to the front brake (outlet) ports. There is a second, independent hydraulic piston that is located inside the master cylinder (down stream) that produces pressure at the rear brake (output) ports. This second hydraulic piston is not mechanically actuated, but hydraulically actuated.

How the FIAT master cylinder works
In order for the second hydraulic piston to be actuated, there must be resistance (back-pressure) from the output ports that feed the front brakes. When the master cylinder is installed in the vehicle and the brakes are applied, hydraulic pressure is sent to the front brakes (caliper and pads) until the pads are completely pushed against the rotor. At this point hydraulic fluid no longer flows to the front brakes and builds back pressure at the master cylinder. This hydraulic back pressure, with the continued pressure applied at the foot pedal, sends hydraulic pressure the second hydraulic piston in the master cylinder to produce pressure at the rear brakes. In other words, the rear brakes DO NOT build pressure until the front brakes have been fully engaged.

Summary--Fix to the problem
With this in mind, and because there is no back pressure from the master cylinder front brake output ports when bench bleeding, it will appear as if the hydraulic piston that feeds the rear brake port is not working. To bench bleed the section of the master cylinder that feeds the rear brakes, the front brake output tubes must be clamped closed to produce the needed back pressure. Another option would be to bench bleed the master cylinder as much as possible (front brakes only) and bleed the rear brakes with the master cylinder installed in the vehicle.

I have a few FIAT manuals and have also looked online in many forums and have never seen this descriptive information provided about the master cylinder. I hope this helps those who find themselves in the same position when bench bleeding a new master cylinder.

Thanks to all who replied to this post.

[DieselSpider]

With about $30 invested in a vacuum bleeder you can eliminate most of these issues.

This one is $19.99 plus $8 shipping on Amazon:



http://www.amazon.com/Brake-Bleeder-Vac ... ds=mityvac

[131]

In order for the second hydraulic piston to be actuated, there must be resistance (back-pressure) from the output ports that feed the front brakes.

So how do the rear brakes work if there is a failure at the front? The whole idea behind having 2 separate circuits.

[laforzaland]

Excellent question (for the engineers who designed this master cylinder)!

[RRoller123]

Pull up on the hand brake lever?

[Roger1200]

If the primary circuit fails, the piston does push MECHANICALLY on the secondary piston applying the rear brakes.

[maxdog]

I've never had much success with vacuum bleeders . I get air bubbles from the bleed nipple threads , rather than fluid and air from the brake system . What's the answer , Teflon tape , special bleed nipples ? Has any one tried pressurising the master cylinder to blow fluid through the system ?
Cheers
Maxdog

[tima01864]

Slowly filled the MC primed it f before attaching the lines first. Gravity bleeding was okay,after i replaced the collapsed center hose. That turned into replacing a broken brake line to the compensator. Messy business. Gravity bleeding did the trick, there was air but patience. Things would of been less messy if i bled out the whole system and replaced the hoses first, Broken bleeders led to new calipers. replacing hoses led to broken brake lines. My 2 cents

[redcars]

I take the bleed screws out and coat them with grease and put them back in. That normally stops the air bubbles from the vacuum bleeder.

[DieselSpider]

I take the bleed screws out and coat them with grease and put them back in. That normally stops the air bubbles from the vacuum bleeder.

Either that or replace the bleeders themselves with the new style designed for vacuum bleeding that have the seals for the threads which many times it just a smear of a LocTite type material that was allowed to dry completely before they packaged them.

Most folks unscrew the bleeders way to far too however it only takes 1/4 to 1/3 of a turn loosening the bleeder to get the job done.