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I have a failed brake master cylinder on my HT Brougham (Disc/drum)
It has a larger reservoir at the front end.
I bought a replacement (also PRB) which looks identical but has the larger reservoir at the rear. The reservoirs are connected at half level in the new one.
will the ports be the same or do I need to swap the lines?
Thanks
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You can only buy the ass-about HZ style like you have purchased. The rear brakes reservoir is at the front so you have to bend the lines to move them. They should have the correct thread size ports that will only go one way, however I have a brand new PBR one that I got when I got the HJ Premier and its ports didn't change whereas the reservoirs did hence it didn't work. |
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Thanks B
I didn't notice the different port sizes until my pipe spanner fitted one and not the other.
The MC I removed had the big reservoir at the front, and the small port at the front. This was connected to the front brakes.
The new MC has the small res at the front and the small port at the front. I have connected this to the front brakes, (no choice without cutting pipes to change fittings).
I now have pedal but the brakes are not effective (I don't think, unless they need bleeding again).
Are you suggesting I need to reverse the brake pipes?
Or get the original MC sleeved & kitted?
Thanks
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AFAIK the small reservoir is rear brakes and the larger one for the fronts and this is why your brakes aren't working properly. I think you need further input from Dr Terry before proceeding. |
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Had a few beers now so I can think.
The MC I bought is a PBR P6613, the ebay vendor Marineauto suggests it should fit HK, HT, HG disc/drum, V8 or 6.
My Monaro (HG) has the same configuration to the one I removed from the Brougham. It is cast iron with the 'big nut' at the front.
The old and new Brougham MC's are bored and assembled from the rear, meaning the front bore cannot be bigger than the rear.
I reckon a disc caliper is going to want higher pressure but less flow to operate, so it wants to be on the smaller bore (front)
A drum w/c needs less pressure but more fluid so it wants to be on the larger (rear) bore.
Based on that logic what I have is correct unless it is for a drum/drum car where the front bore would be smaller than the rear, but not as small as a disc bore.
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In the standard setup the front master cylinder reservoir is for the front brakes - discs in your case. It has the smaller pipe fitting. The picture below may help, and shows three types of master cylinder - from front to back, original HT "big nut", NOS HQ "big nut", and PBR P6613. All have the small pipe fitting at the front. Perhaps the PBR P6613 has changed, as this one is from around 1987, but I was under the impression that they were a drop in replacement for HKTG without swapping pipes around. My understanding is that discs require more fluid flow than drum wheel cylinders, but at lower pressure due to the larger piston surface area in a caliper. (Force = pressure x area)  Edited by user Saturday, 2 May 2015 3:46:09 AM(UTC)
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All these master cylinders have the same bore all the way through, 1-inch.
There is no difference in pressure or flow between the front & rear brake systems.
The reason for the larger reservoir is that the fluid level drops over time in the reservoir connected to the front discs, as the pads wear & the pistons extend. With the rear drums, baring leaks, there should be no fluid drop.
In early 1977, GM-H changed the m/cyl to the much larger casting to comply with ADR31 & the upcoming 4-wheel dis brake system & for whatever reason changed the front / rear pipe fitting arrangement.
At one time PBR made replacement alloy m/cyls to suit both versions (small pipe/large pipe fittings swapped), I'm sure both are still available, I'll check my catalogues on Monday.
If it was my car, I would get the big nut m/cyl re-sleeved in stainless & rebuilt. A much better long term solution. I do exchange units if required.
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Terry, I'm a little confused. If the pressures are the same front and rear, (and doesn't this depend on the accuracy of the ratio of the tensions of the springs in the master cylinder?) how can the flow, and hence the volume of fluid displacement be the same when the volumes of the front and rear systems are different, even for an all drum system, where the front wheel cylinders are larger bore than the rears?
The only way I can see for the displaced fluid front and rear to be identical is for each hydraulic circuit to actuate 4 identical wheel cylinders by exactly the same amount.
What have I missed?
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You are on the money gm5735, there will be a difference in volume/displacement between front & rear, but in the real world it's actually 2/10 of f'all. It's all calculated by the design engineers & only causes dramas when the rears become badly out of adjustment & more volume/displacement is required for the rear cyls.
Any small differences are taken up by the springs separating the front & rear pistons in the m/cyl. It's one of those compromises that has stood the test of time.
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