Rear brakes locked up ?

Col-b

Club Member
Hi guys i just serviced the car new oil, filter, plugs etc so took her out for a test drive. She was going like stink i was grinning like a Cheshire cat at how well she drove. Then i pulled up at the garage that did the body work as it was on my way home no one was in so i got back in the car to drive home and the rear brakes just seem to lock on when i started the engine and the brake pedal was rock hard. I waited 20mins started her up and they seemed to have freed up. The journey back was about 4 miles. I tried to just use the gears to slow down and applied the handbrake a couple of times but about a mile from home i could feel the rear brakes binding up and the car struggling to drive. I made it on to the drive, hope i haven't burnt out the clutch. After 2 hours i started her up again and drove into the garage no probs ? Any idea whats going on guys and gals ? I have a Wilwood master cylinder to fit so i will do that and i have a wilwood proportion valve. I just searched some old threads and also came across combination proportioning valve too.
http://www.rallydesign.co.uk/product_info.php?products_id=15436
My question is although i have a 240Z i have the later 260Z prop valve fitted to the bulk head if i by pass this completely can i just join the in and out of the front brakes and put the wilwodd prop valve between the in and out of the rear brake line ? And can you delete the dual pressure warning valve on the inner wing.
Or would it be better to delete both old valve units and fit the combination valve in the link above ?
Any advice and pictures of work carried out would be much appreciated.

thanks

Colin
 

atomman

Club Member
Have you put new brake flexis on the back ?

If they are old they may have collapsed not letting the fluid back through , locking up the rears ,

When you fit the Wilwood MC also make sure you take out the valve in the back circuit if you are keeping the drums other wise they will lock up again ,
 

Col-b

Club Member
Have you put new brake flexis on the back ?

If they are old they may have collapsed not letting the fluid back through , locking up the rears ,

When you fit the Wilwood MC also make sure you take out the valve in the back circuit if you are keeping the drums other wise they will lock up again ,

I will check them out. I thought you had to take the valves out the front ones and leave them in for the rear drums ?
 

Col-b

Club Member
No idea about the valves mentioned but sounds like typical flexi collapse to me.

this is what i found on another forum
The port to the master cylinder reservoir isn't opening when you let off the brakes.

This is either caused by the pedal adjustment being too tight (should be 1/8" or so free play in the pedal before the master cylinder starts to move), or the rod in the booster is too long.

When the master cylinder port opens to the reservoir, it releases whatever pressure might be in the system. If it cannot open then as you drive the fluid gets hot and expands, this is like stepping on the brakes a little tiny bit. This causes the fluid to get hotter and it expands more which applies the brakes more and creates more heat etc and is a vicious cycle until you pull over with smoke coming out of the wheels. If you cracked a bleeder open at that point, it would squirt out like someone was standing on the pedal.

Get some free play in there and you'll be good to go.
 

jonbills

Membership Secretary
Site Administrator
There's a thingy in the MC that takes the pressure off the rear circuit so the springs on the shoes can pull the shoes off the drum, because drum brakes pull themselves onto the drum with any contact. (and the adjustment ratchet keeps it there)
 

Turn & Burn

Club Member
this is what i found on another forum
The port to the master cylinder reservoir isn't opening when you let off the brakes.

This is either caused by the pedal adjustment being too tight (should be 1/8" or so free play in the pedal before the master cylinder starts to move), or the rod in the booster is too long.

When the master cylinder port opens to the reservoir, it releases whatever pressure might be in the system. If it cannot open then as you drive the fluid gets hot and expands, this is like stepping on the brakes a little tiny bit. This causes the fluid to get hotter and it expands more which applies the brakes more and creates more heat etc and is a vicious cycle until you pull over with smoke coming out of the wheels. If you cracked a bleeder open at that point, it would squirt out like someone was standing on the pedal.

Get some free play in there and you'll be good to go.[/QUOTE

This is exactly where I’d be looking, the rear brakes locking is fluid trapped between the rear brakes and the Master Cylinder. This can be caused by the piston in the MC sticking in a brake pedal pressed or forward position. Not sure why it would stick it could be corrosion within the MC or poor adjustment between the Servo and the MC, or possibly even bad pedal adjustment. The solid brake pedal feel backs this up.
There may be some mechanism for the prop valve to create this fault also, this contains pistons and I think small springs.
On your new wilwood setup you can remove the old the prop valve and use the adjustable one on the wilwood. I’ve taken the same route.
 

Col-b

Club Member
This is exactly where I’d be looking, the rear brakes locking is fluid trapped between the rear brakes and the Master Cylinder. This can be caused by the piston in the MC sticking in a brake pedal pressed or forward position. Not sure why it would stick it could be corrosion within the MC or poor adjustment between the Servo and the MC, or possibly even bad pedal adjustment. The solid brake pedal feel backs this up.
There may be some mechanism for the prop valve to create this fault also, this contains pistons and I think small springs.
On your new wilwood setup you can remove the old the prop valve and use the adjustable one on the wilwood. I’ve taken the same route.
Did you by pass all the old Datsun valves including the brake fault one and just use the Wilwood products ? Any pics of your set up would be great thanks
 

Turn & Burn

Club Member
These are only 3 I’ve got that are any use. You can see the rear brake line goes direct to the bias valve and then heads to the rear of the car. The front line drops to a tee where it splits to each side of the car. Tbh I don’t think I’ll need the bias valve, I’ve not checked the rear braking force yet but I think the front brakes (4 pots on 300mm vented discs) will have about 70% of the braking force.7A8541BD-3F6D-4055-BE0F-474A6E8C67D3.jpeg6629E362-1647-4CA1-8F80-045884238FAD.jpeg32A3DD14-FB22-4989-85A1-7646E3DE80F4.jpeg
 

moggy240

Insurance Valuations Officer
Staff member
Club Member
IMG_20210227_173210.jpg I just used a Wilwood Combination Proportioning Valve which also has the brake light warning switch but I don't use that bit. The only thing with the wilwood stuff is its imperial fittings.
 

Col-b

Club Member
These are only 3 I’ve got that are any use. You can see the rear brake line goes direct to the bias valve and then heads to the rear of the car. The front line drops to a tee where it splits to each side of the car. Tbh I don’t think I’ll need the bias valve, I’ve not checked the rear braking force yet but I think the front brakes (4 pots on 300mm vented discs) will have about 70% of the braking force.View attachment 43805View attachment 43806View attachment 43807
thank you that really helpful
 
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