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Have a 1991 Z-28 with 4-way disc brakes (following a rear brake conversion). All else is stocked including its 5L and 10-bolt rear. Do I need a Master Cylinder &/or Proportioning Valve Upgrade? Have any part recommendations? Keeping it stock so no need for high performance parts. Also note, even prior to the brake conversion, brakes were always spongy requiring pedal to be basically at the floor for stopping (after being bled). Would like thoughts on that as well. Thank you!
I also have a '91 Z28 which I converted to rear disc. I swapped the proportioning valve with the correct part number for a four wheel disc car, which I believe is 10164112 (you should double and triple check that). Still using the stock master cylinder, as I believe they were the same for drum and disc cars.
The spongy brakes could be any number of things, but one thing that tripped me up during my swap was failing to bench bleed the master cylinder properly. I didn't get it perfectly horizontal, so there was still air in the system. You might also want to check the flexible lines between the calipers and the hard lines on the body for any damage or leaks.
Thx BovineZro! I have ordered the p/v. The reason I asked about the master cylinder was due to the spongy brakes. No damage or leaks on any of the brake lines. However wondering if the issue could be in the master cylinder, booster, diaphragm, other...??? Thank you!
I'm no expert (far from it), but my general understanding of our brake systems is that 99% of the time spongy pedal feel is either a leak or air in the system.
The reason I asked specifically about the flexible lines between the body and the calipers is because the rubber lines can fail over time and start to bulge when pedal pressure is applied. That can give you spongy pedal feel. It can be hard to see those bulges - you'd almost need to have someone in the car pressing the brake pedal while you check each line.
A bad master cylinder (or one with air in it) and/or a bad booster can cause similar symptoms. If you've got things apart to swap the proportioning valve, it wouldn't hurt to bench bleed the MC and replace or rebuild the booster, especially if they're original to the car.
Would either of you happen to know the fitting size for the front lines where they go into the prop valve? I have a 91 prop valve as well. I understand there were two metric prop valves for disc brake cars and they have different fitting sizes. The iron caliper prop valve uses M10-1.0 fittings for the front line to prop valve connection. The PBR prop valve uses a coarser thread fitting. I tried to see fi the the front MC to prop valve line M10-1.5 fitting would fit and it doesn't. So maybe an M10-1.25?
QwkTrip - I have that same diagram from my research, probably from your post. It looks like that info was based on your 89. I have a 91 prop valve specifically for a PBR equipped car (broadcast code "LF" in your prop valve chart). The new brake lines I bought from Inline Tube with the M10x 1.0 fittings do not fit. The 91 prop valve appears to use a coarser fitting for the front lines.
Item #10164112 is the GM OEM prop valve item and references AC Delco's item #172-2074. I am not having any luck finding a place that stocks/sells this p/v...anyone have any ideas?
1989 was the transition year for fitting sizes so an 89 could have either one depending when it was built.
You don't have to guess or even trust what somebody says here. Just get an assortment of bolts or fittings and see what threads in properly. Or use a thread checker tool if you still have the original fittings.
Item #10164112 is the GM OEM prop valve item and references AC Delco's item #172-2074. I am not having any luck finding a place that stocks/sells this p/v...anyone have any ideas?
Ahh yeah sorry, forgot to mention I picked mine up used from Hawk's several years ago - it wasn't new. Generally you'll pay a premium with Hawk's vs eBay, but of course eBay is sketchier.
Nice, I'd be curious to know how that one works for you! At that price I might go ahead and buy one, then stick it on a shelf in case the one in the car fails.
Do you (or someone) happen to have the size of all 5 prop valve fittings on a 90-92? I prefer to purchase brake line fitting adaptors and use this prop valve. Here is the valve with current fitting sizes along with the adaptors:
there are no adpapers made to fit the M10-1.5 (i spent 2 days looking) and calling.. if you flair your own lines to inverted or what it out there for sale.or you can use the line nuts from your factory lines you Could do that..thats what Im doing..The tool is $40 then you can swap it. I had to buy the tool and line to hook my line lock up 100% right.. 30 years ago I just flared the factory line and ran it in. with small brass cups...it never leaked .. now I want to be 100% right..
Last edited by articwhiteZ; Jun 17, 2021 at 07:50 AM.
with this tool you can do the lines on your car.. to ISO or inverted .. and your good to go.. the first part of doing a inverted flair.. (what the tool was mad to do) comes out to a ISO bubble. so ya stop there and you good.. to do the the inverted.. ya just go to the 2nd step with the tool.. easy!.. this one is only for 3/16 tube. Front brake lines.
I have a flaring tool, but that one is really cool and more portable...I may just get that one! Do you happen to have the nut sizes for all 5 prop fittings? The front may be M10-1.5, but there are several other sizes of the 5 and I wonder if any are the same as the diagram I provided above...
Check the vacuum hose to the brake booster and then check your engine's vacuum. If you're not pulling at least 15 pounds, your brakes won't work so well, if at all. Mine didn't.
Thx @Johnny Gun. Should the pull be under 15#, what is the fix?
The fix for me was a whole new motor, but you probably don't want to go that way. Mine was so worn out I was getting barely over 100 psi in the cylinders.
Haha! Sorry misunderstood. Engine vacuum fine. When you say "check vacuum hose" are you referring to check for holes, leaks, cracks or remove and check for occlusions or other?
Haha! Sorry misunderstood. Engine vacuum fine. When you say "check vacuum hose" are you referring to check for holes, leaks, cracks or remove and check for occlusions or other?
By the way.. it so much faster and it easy to make your own brake lines..been working with the tool to do the metric lines.. sorta fun in a way..just using thr Factory line nuts on the new SS lines..one end is Metric ISO flair ..(bubble) and the other end is SAE inverted flair...
