Ferrari 308 QV Brake System Nightmare! Need Some Help rear circuit innoperable

Hi guys this is my first post on here and really need some advice from other 308/328 owners on this situation I am dealing with.

I bought a Girodisc brake upgrade kit which uses 2 piece rotors and front Wilwood 4 piston calipers. I use their stuff all the time with great success. The kit also includes 4 stainless flex brake hoses that attach to the original hard lines and you reuse the ATE rear brake calipers with the parking brake lever from the Porsche 914.

The front Wilwood calipers connect the new flex line to the factory hard line above the steering rack. We do not use the small caliper hard lines the original front calipers required.

Out back the calipers are original and using the original small hard lines to connect to the new stainless flex lines

Replaced the master and booster made all the correct adjustments to push rod etc and also replaced the pressure safety valve under the car.

Bled the system multiple times via Motive and vacuum and gravity and still no luck after 6 liters passing trough. The front circuit works great however the pedal travel is too low and the rear calipers do not stop the rear wheels. There appears to be a pressure drop out back that is making the rear calipers useless.

We adjusted the rear caliper pistons via the mechanical adjustments per shop manual using a feeler gauge.

After various diagnostic tests of putting PSI gauges at the master and all circuits there is nothing wrong with the master or booster. the front circuit is perfectly fine. Zero leaks. When I bypass the rear rebuilt calipers and put M10 bleeder screw to plug up both rear stainless flex brake lines the brake pedal gets hard as a rock as it should when the full system is operational. The issue I think is these damn calipers from hell out back.

this leaves me to believe the NOS calipers I bought are somehow defective and leaking from the spacer seams in the middle causing a huge pressure drop and soft pedal. I can not tell if these castings have hairline cracks in them as they were sent to PMB for restoration. I bought a set of NOS cores last year on Ebay and sent those in for restoration. Since the calipers are cad plated nicely it is hard to see leaks but I do feel a slickness to the body of the caliper housing on the backside where the pads load which means they are sweating brake fluid somehow but no puddles or leaks on the floor..

has anyone else had this issue and is this something that I can fix myself with a simple caliper rebuild kit or do i need new calipers as these might be compromised and have issues.

I have been searching for an alternative rear caliper to even eliminate the parking brake but can not find anything for these cars as an upgrade for the rear caliper.

Any input or advice is much appreciated as this is a very simple system that has been giving me absolute torture for weeks now

 

Thanks for the replies i made many calls and it seems to all point to the rebuilt rear calipers. I might be forced to buy another set and they are not cheap but not much of a choice as two specialist did say my cores might have been messed up by a hack rebuilder.’’all components in the brake system are brand new minus questionable rebuilt rear calipers. One guy said if the adjustment lever for the parking brake is very stiff and if the parking brake doesnt work then the internal mechanism may be sheared and when this happens it causes the fluid to seep out of areas and a massive pressure drop. These calipers out back seem to be a pita if someone lacks the experience to rebuild this style.

 

Hi this is a EURO car and it have what they call a Brake Safety valve up front by the rack and pinion which was just replaced due to rust. So this valve up front is a distribution valve and has some sort of a check ball inside for flow and to trigger the brake warning lamp on the dash. In the rear frame of the car is just a basic brass T where the main line from the front connected and branches off to both rear calipers.

 

I bench bled the master 5 times with this very nice kit from Thexton which uses check valves on the hoses and aluminum fittings unlike the cheap plastic kits they sell at NAPA. The issue now seems to be issolated directly at the rear calipers themselves because I capped off the rear flex hoses and the pedal is hard as a rock now, once I reconnect the rear calipers not only do the brakes out back not work but the pedal travels way down, I think I need to bite the bullet and buy new rear calipers I have a feeling the rebuilder messed these up after speaking with a specialist yesterday on this in detail

 

This is the professional master cylinder bleeding kit I used this thing really was worth the money
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Here is the brake valve up front under the car that was replaced they call it a brake pressure safety valve

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Peter this valve you posted above looks different than what I replaced where exactly is this mounted as my car is an EURO model, the only valve I found was up front which I just posted a photo in this thread. I do not see a secondary valve on the factory diagram illustrations just the one up front and a rear brass 3 way T fitting on the drivers side rear chassis rail inward

 

Upon Peter posting the brake bias valve photo above I did a search now on Maranello site Diagram 31 and see it in the photo diagram but not sure if my car does or does not have this valve of if this valve is for a much earlier car.

I replaced valve #22 which they are classify as a Brake Warning Switch, Valve #13 is a Brake Bias Valve Rear Circuit I was under the assumption that the part I replaced did more than one function but maybe I am wrong and I need to replace #13 if my car even has this.

Can someone post a photo or location to where #13 location my eyes are getting worse and I never saw that under my car but now need to take a second look tonight when I get back

 

FYi so the calipers sweat fluid it is hard to see because of the plating but if you touch them they are wet but no puddles on the floor. This might be coming from the halves matting together or from the lever area for the parking brake as that can be a source of a leak, i did send them back twice and the exact same thing is happening so I completely lost faith in this. I have spoken to other specialists who deal with these style calipers and learned quite a bit so far over the phone that things can go wrong on a rebuild. i checked both calipers now and there is fluid on the calipers in the same spots near the pads. I am thinking maybe the castings on my cores are defective (Ebay purchase)or they have some sort of hairline crack and the EBay seller never disclosed this. I just bought another set of calipers today rebuilt from a reputable vendor who will honor them to work correctly i will try them out this weekend hoping it solves this issue. Also my front brakes work perfectly fine I can drive the car around fine but I really want my rear brakes to work for safety and panic. The pedal gets lower with the rear calipers connected but never to the floor just low as if it is loosing pessure per pump on the rear circuit which indicates a leak somewhere in the system. This is why i did testing with capping off the rear lines and bingo the pedal is nice and firm. Oh I also bought an SSBC brake pressure testing kit from Jegs and removed the passenger rear caliper bleeder and connected the gauge. I was getting from 300-500-200 PSI on each press of the pedal which means there is a pressure leak just enough to cause this. This has been beyond frustrating to diagnose and track down and with old car and buying used parts it can always be a gamble what will happen.

 

Thanks Peter I just looked under the car so all the lines connect into the brake safety switch valve and only one line travels through the chassis then comes out under the rear driver fuel tank area to the rear frame rail to the a brass T and they branch off to each rear caliper. I did not find that other brake proportional valve you posted above on this car so it must be for the older models.

 

thanks for posting this photo what year is your car this is not on my car as the line comes from the brake switch and goes straight into the frame to the rear

 

Guys can anyone here confirm the exact brake line flares that these Ferrari 308 use for the caliper hard lines? I want to make new lines for the rear when I install the calipers so I know everything is perfect. today i saw a drip from the calipers coming from the halves i put a blue shop towel down last night and today I finally saw the leak from the case halves.

in regards to the flares I have the Eastwood turret style flare kit and the Hydraulic kit, both kits are confusing with the DIN ISO flares, the hydraulic kit only has a 4mm ISO and the turret kit has a 37 degree bubble and a 45 degree DIN.
anyone who can confirm tonight would be great as I plan to work on the car tomorrow all day