1971 Olds Front disc brake conversion. New booster and cylinder and proportioning valve. Can not get pressure to rear drums after bleeding them.

A Proportioning valve is required on vehicles that have disc brakes on the front wheels and drum brakes on the rear wheels. Disc brake pads are normally in contact with the disc, while the drum brake shoes are normally not in contact with the drum. ... The proportioning valve reduces the pressure to the rear brakes while keeping more pressure on the front brakes. Thus, the front brakes do more stopping than the rear.

no
rear wheel brakes operate at different pressures because of the proportioning valve in the lines
you are correct that failing brake hoses can fail internally and lock the brakes on but I feel that that is not your problem
This is how disc brakes work
There is no check valve in the master cylinder as there is in a car with drum brakes so any fluid captured by pedal ( the exception here is if the car has read drum brakes in which case that section of the master cylinder will have a check valve )movement is pushed into the calipers and applies the brakes
Now on releasing the pedal , that fluid is allowed to return to the reservoir by a compensating port in the master cylinder
That port blocks up and the disc brakes remain on.
heat build up from the brake pads expands the fluid and so pushes the pads harder onto the disc( Lock on that you release by bleeding the calipers)
SO the problem will be in the master cylinder
having the brakes professionally flushed may relieve the problem for a period but a new master cylinder fitted professionally will fix the problem

Use a pressure bleeder.

Pretty normal to not be able to lock the brakes without the assistance of the booster, it'll take a lot of pedal pressure without it. It's also normal for the pedal to feel harder without the booster, this goes for any car/truck with power brakes. Try it with any other car you have and you'll see.

I've worked on several old Cadis but don't recall what year they started using front disc. If it's been changed from drum to disc you'll likely need a different proportioning valve and possibly a different master to get them to work right. Talk to the seller and ask if the brake feel changed when they replaced the master and if they changed any other parts.

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BY brake fade away I take it that you mean the pedal goes to the floor after you taken your foot off the brakes. . The valve you left out is a compensator valve that proportions the pressure between front and back to stop rear brake lock up and side skidding. I think the problem is in the master cyl in that the nonreturn valve under the second piston is not working and hold residiual pressure in the rear lines (Assuming they are drum brakes). If not you will need to bleed the m/cyl by cracking the lines at it one ata time and emptying the cylinder of trapped air allowing a full charge of fluid every time you pump the brakes.

try bleeding your ABS block first, if that doesn not work could need a master or proportioning vavle. Some scan tools are capable of bleeding the ABS system on some vehicles, and some you have to bleed manually.

to bleed the brakes & should be done w/proportioning valve in place - you start at farthest corner - right rear, left rear, right front and left front.... If you suspect proportioning valve you can pick them up cheap at any salvage yard.

here is a 96 taurus


Item Part Number Description 1 2C296 Anti-Lock Brake Tube 2 2C219 Anti-Lock Brake Module 3 2005 Power Brake Booster 4 2263 Front Brake Tube 5 2B091 Brake Pressure Control Valve 6 2140 Brake Master Cylinder 7 2B547 Brake Load Sensor Proportioning Valve 8 N802191-S60 Bolt 9 2L568 RH Rear Brake Tube 10 2L569 LH Rear Brake Tube 11 2C287 Brake Fluid Distributor Tube 12 N808531-S301 Bolt 13 — Front Side Member 14 N802324-S56 Bolt 15 18124 Front Shock Absorber 16 2B121 Front Disc Brake Caliper 17 2078 Front Brake Hose 18 2264 Front Brake Tube 19 2553 Rear Disc Brake Caliper 20 2A442 Rear Wheel Brake Hose 21 N808728-S60 Bolt 22 — Side Rail 23 N802324-S60 Bolt
Brake Load Sensor Proportioning Valve


Item Part Number Description 1 2B547 Brake Load Sensor Proportioning Valve 2 — Body 3 N802191-S56 Bolt (2 Req'd) 4 2L568 Brake Tube Assy 5 2L569 Brake Tube Assy 6 2265 Brake Tube Assy 7 2B255 Brake Tube Assy 8 5500 Rear Suspension Arm and Bushing 9 W611634-S301 Bolt A — Tighten to 11.4-15.6 Nm
(8-12 Lb-Ft) B — Tighten to 10-14 Nm (7-10 Lb-Ft)

Brake Hose, Rear —Drum


Item Part Number Description 1 — Rear Side Rail 2 2L569 Rear Brake Tube 3 N602324-S60 Screw 4 N611193-S60 Screw 5 2C023 Left Hand Rear Hose and Connector 6 2210 Rear Drum Brake Backing Plate 7 18125 Rear Shock Absorber 8 2267 Rear Brake Tube A — Tighten to 16-20 Nm (12-15 Lb-Ft)

The proportioning valve most definately can be the problem. Did you change it or is it the original? (A disc/drum system uses a 70%front/30%rear braking ratio, whereas an all-disc system is alot more even). I would first bleed the master cylinder. Then I would 'gravity bleed' the rest of the system by opening the bleeders on all 4 wheels. Do not at any point let the m/cylinder go dry. Also, do not touch the brake pedal.
Continuously check all 4 bleeders, when clean brake fluid with no bubbles at all is running from a bleeder, close it. When all 4 wheels are done, recheck the fluid level and you should be OK.
Pump the pedal to 'self-adjust' the rear calipers, then re-check the fluid level.
Any problem after that means you either have a loose connection (it's possible for a tiny leak to let air in, but not let fluid leak), a leak, or you need a new (disc/disc type) proportioning valve. I would try Summit Racing myself. Good Luck!

The Chardes have a diagonal split system, so one of the front discs is plumbed to the diagonally located rear drum.

You may need to check the front brakes too and that they have been bleed properly.

Other common cause of uneven braking are:

seized wheel cylinder;
brake hose obstruction or weakend wall;
weak pull off spring on shoe;
uneven drum roundness;
compensator/proportioning valve dicky.