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I'm surprised by that because opgi states "w/ Disc Brake Master Cylinder" and my car has disk brakes.
Also that is the same cylinder as the old one in the car right now.
I have no buildsheet but lets say the disk brakes are stock, i would think the cylinder i bought and the old one currently in the car are correct for this vehicle?
All i know is power & disk brakes were optional on my car: https://www.lov2xlr8.no/brochures/olds/72olds/21.html
Lets say someone butchered something together.
Could it really be that i need the other master cylinder?
Or do i and the last owner miss something and i really need the master cylinder with the smaller bore diameter?
This is from the dorman website regarding the master cylinder the shop thinks is correct for my car
"1972 Oldsmobile Cutlass Supreme Front Drum Brakes Bore: 1 In." https://www.dormanproducts.com/p-31930-m71277.aspx
So since my car has the disk brake option i think the shop is full of crap?
The original disc front brake master cylinders have a 1-1/8" bore so the shop is completely wrong in that regard.
There is a rod/"valve" setup in the proportioning valve that may have moved into a position that is affecting the bleeding process....it's the part of the valve that is meant to shut off fluid to either the front or rear brakes in the event there is a leak in the system, broken hose, etc.
There is a cheap tool made that is installed temporarily into the proportioning valve to hold this mechanism in a position so bleeding can take place and it prevents movement of the mechanism that would otherwise block off fluid flow to the front or back half of the system.
Last edited by 70Post; July 14th, 2022 at 09:31 AM.
The dorman master cylinder you listed is for 4 wheel drum brakes. Your original master cylinder should have looked like this.
The larger front reservior is for the larger pistons in the calipers (more fluid volume to fill) vs the wheel cylinders in back. The one you bought should work as well though. You said the shop replaced the proportioning valve. Why exactly did they do that and what did they replace it with? The original valve for a 72 with disc brakes would look like this:
They could have replaced it with one like this but these come in disc/ drum and 4 wheel disc versions:
As long as the system has been properly bled (are you sure it is), the master cylinder you bought is most likely not your problem. More likely the rear brakes are not adjusted properly. Is the shop firmiliar with how to adjust drum brakes? Or the proportioning valve is wrong. Again, why did the shop replace the proportioning valve that was on the car?
Last edited by Loaded68W34; July 13th, 2022 at 06:34 PM.
They replaced it because they thought it's causing the poor rear brake performance.
You should think they know how to adjust drum brakes since they restore & repair these cars since the 80s but i'm not a huge fan of this shop.
But my dad thinks they're good (i can't convince him they're not) and we need them to pass the required inspection every couple years. (TÜV)
That valve, master cilinder, and booster should work fine with factory calipers and wheel cylinders. I assume they are getting good flow from the rear while bleeding correct? If not, it could be a clogged line. Did they say specifically why they think there is poor performance from the rear brakes? As in, what did they say the brakes are doing? Are the fronts locking up and the backs are not under full braking? Is the car on the lift and the rear wheels are not getting any resistance while pressing the pedal? It is worth noting that the factory disc brake system was designed to do most of the braking up front.
Your original master cylinder should have looked like this.
That's exactly how it looks. (The old one still in the car)
Originally Posted by Loaded68W34
That valve, master cilinder, and booster should work fine with factory calipers and wheel cylinders. I assume they are getting good flow from the rear while bleeding correct? If not, it could be a clogged line. Did they say specifically why they think there is poor performance from the rear brakes? As in, what did they say the brakes are doing? Are the fronts locking up and the backs are not under full braking? Is the car on the lift and the rear wheels are not getting any resistance while pressing the pedal? It is worth noting that the factory disc brake system was designed to do most of the braking up front.
I don't know if they tested the brake lines and hoses or if they even bothered replacing the wheel cylinders.
For now i just wanted to be sure that i'm right and they're wrong regarding master cylinder choice. And that seems to be the case.
Just one more question. Lets say my car had 4 wheel drum brakes from the factory and someone upgraded to front disks.
Could they have butchered something together that really requires the drum brake master cylinder or not?
Last edited by Michael_; July 14th, 2022 at 09:41 AM.
They could have butchered together just about anything as long as things would bolt up.....but that means nothing. The functionality/performance of whatever was butchered together would likely be less than optimal. Thus....using the correct sized master cylinder, etc for front disc brakes. Front discs require the correct sized master cylinder for the whole system to work optimally.
They could have butchered together just about anything as long as things would bolt up.....but that means nothing. The functionality/performance of whatever was butchered together would likely be less than optimal. Thus....using the correct sized master cylinder, etc for front disc brakes. Front discs require the correct sized master cylinder for the whole system to work optimally.
So you're basically saying no matter what has been potentially done there is no way a 4 wheel drum brake master cylinder would perform better with a front disks / rear drums setup, right?
You do not want a cobbled up brake system with by guess and by golly subpar aftermarket parts. Send your original Master Cylinder to White Post Restorations for a rebuild. I also had limited/no flow at the rear wheels. Turns out the rubber brake hose on top of the differential had collapsed internally. Looked fine outside, but inside was clogged.
What did the factory do/design/specify? (Hint: they didn't do what they did just for grins). Do you want a sub-par brake system?
Ya know i'm not an expert so the reason for me asking was just to rule out the shop might be actually right.
But now i know that using a 4 wheel drum brake master cylinder instead of a disk/drum combo master cylinder does not make any sense on a disk/drum system no matter the exact setup.
Ergo: The shop is wrong without a doubt.
Thanks for letting me know that!
BTW: I do not expect the brake system to be modified, but i don't know for sure.
Originally Posted by 4+4+2=10
I also had limited/no flow at the rear wheels. Turns out the rubber brake hose on top of the differential had collapsed internally. Looked fine outside, but inside was clogged.
July 13th, 2022, 02:41 PM
