Hey there,

After finally getting my drum brake system upgraded (MPB dual 9" booster and dual master cylinder), the car stops amazingly-there is so much more power than it had.

However, there is an issue that seems to be confounding everyone...

The rear brakes build pressure while driving, the LHR shoes eventually contacting the drum. RHR seems to also, but not to the same degree. When you pull up, the left side is smoking. Temps as measured at the backing plates immediately after a drive:- LHF 45-50C, RHF 45-50C, LHR 130-140C, RHR 90-100C.

Car has been back to the installer who is a good guy, and brake specialist-did all my brakes up when I bought the car (rebuilt wheel cyl's, rebonded shoes, radius ground drums, replaced a coupe of lines, etc). He has to date rebonded rear shoes and ground rear drums again (LHR shoes were cooked, obviously), rebuilt wheel cyl's, replaced flexible line at rear, removed, checked and reset booster and rod. All hard lines have been renewed. So, all that can be done seems to have been addressed, yet the problem persists.

Any suggestions, advice?

 

 

 

Is the short pushrod between the brake booster and the master cylinder a wee bit too long? I had this problem on my 70 El Camino and one each of a washer on the posts between the back of the master cylinder and the booster solved this problem.

 Trim / grind a flat edge onto each washer to hide the washer and nobody but you and your mechanic will ever know they are there.  Except us, of course.

Rick 

22 hours ago, B52bombardier1 said:

Is the short pushrod between the brake booster and the master cylinder a wee bit too long? I had this problem on my 70 El Camino and one each of a washer on the posts between the back of the master cylinder and the booster solved this problem.

 Trim / grind a flat edge onto each washer to hide the washer and nobody but you and your mechanic will ever know they are there.  Except us, of course.

Rick 

Hey mate,
Thanks for your reply.

I think they've adjusted the rod length a few times, but I'll check this solution with them.

Paul 

make sure the self adjusters on your factory set ups are locking … if they don’t they can be winding them selfs up slowly 

Also 

you need to give us more info 

I see a master cylinder .. but does your system have a preportioning valve ? 
if so it may have a residual valve that’s not letting the fluid return to the master cylinder hence keeping a certain amount of pressure in those rear brakes 

and here’s another little clue … does the rear brakes behave on short runs but on long runs cause the issue ? 
If so 

it’s a residue valve sticking because it’s getting hot from its location to the manifold ! 
been there had that happen ! 
more close up pics will help . I can’t see from that one pic you have shown 

And justA to be clear … you have only changed your master cylinder and booster correct ? What was there before ? JustA manual master cylinder ? 
all your brakes are stock drums right ?

Paul 

I have justA had a read on mp brakes web site and I can see what you have bought 

you need to confirm with them that your master cylinder is the correct one for drum/ drum set up 

On 6/18/2025 at 7:26 PM, 64 kiwi boni said:

Paul 

make sure the self adjusters on your factory set ups are locking … if they don’t they can be winding them selfs up slowly 

Also 

you need to give us more info 

I see a master cylinder .. but does your system have a preportioning valve ? 
if so it may have a residual valve that’s not letting the fluid return to the master cylinder hence keeping a certain amount of pressure in those rear brakes 

and here’s another little clue … does the rear brakes behave on short runs but on long runs cause the issue ? 
If so 

it’s a residue valve sticking because it’s getting hot from its location to the manifold ! 
been there had that happen ! 
more close up pics will help . I can’t see from that one pic you have shown 

And justA to be clear … you have only changed your master cylinder and booster correct ? What was there before ? JustA manual master cylinder ? 
all your brakes are stock drums right ?

Hey there,

Yes all standard - drums, shoes, wheels cylinders all standard.

The system is exactly as it was before the master cylinder and booster were changed, and the issue has only arisen since the change.

The problem is a little sporadic, but has shown up on both longer and shorter trips, so heat build up isn't suspected at least.

I believe the mechanic has ensured the set up is correct, but will double check.

Oh - no proportioning valve, but that affects the proportion of fluid between front/rear, affecting the brake performance doesn't it? In this case the brakes work amazingly, but the pressure builds up gradually.

Edited June 23, 2025Jun 23 by Bonne61

9 hours ago, Bonne61 said:

Hey there,

Yes all standard - drums, shoes, wheels cylinders all standard.

The system is exactly as it was before the master cylinder and booster were changed, and the issue has only arisen since the change.

The problem is a little sporadic, but has shown up on both longer and shorter trips, so heat build up isn't suspected at least.

I believe the mechanic has ensured the set up is correct, but will double check.

Oh - no proportioning valve, but that affects the proportion of fluid between front/rear, affecting the brake performance doesn't it? In this case the brakes work amazingly, but the pressure builds up gradually.

That sugests there is a residual valve not letting pressure back into the resour Paul

On 6/23/2025 at 6:20 AM, Bonne61 said:

Hey there,

Yes all standard - drums, shoes, wheels cylinders all standard.

The system is exactly as it was before the master cylinder and booster were changed, and the issue has only arisen since the change.

The problem is a little sporadic, but has shown up on both longer and shorter trips, so heat build up isn't suspected at least.

I believe the mechanic has ensured the set up is correct, but will double check.

Oh - no proportioning valve, but that affects the proportion of fluid between front/rear, affecting the brake performance doesn't it? In this case the brakes work amazingly, but the pressure builds up gradually.

Paul, what Kiwi said! All drum brake master cylinders use residual check valves. While the purpose is to keep the shoes in light contact with drum so you have a firm pedal on the first depression of pedal. They can create issues! They also come in different residual pressure values. So, I would suggest this! They are not difficult to remove or reinstall. So remove the back set. Bleed them & see if the issue improves. Be careful & aware that upon reapplication of the brakes you may have a softer pedal, but only about 30 percent of your braking comes from the rear brakes. I could go into more detail, but this should give you more than enough to determine if this is the issue.

On 6/23/2025 at 3:49 PM, 64 kiwi boni said:

On 6/23/2025 at 6:20 AM, Bonne61 said:

Hey there,

Yes all standard - drums, shoes, wheels cylinders all standard.

The system is exactly as it was before the master cylinder and booster were changed, and the issue has only arisen since the change.

The problem is a little sporadic, but has shown up on both longer and shorter trips, so heat build up isn't suspected at least.

I believe the mechanic has ensured the set up is correct, but will double check.

Oh - no proportioning valve, but that affects the proportion of fluid between front/rear, affecting the brake performance doesn't it? In this case the brakes work amazingly, but the pressure builds up gradually.

On 6/23/2025 at 3:49 PM, 64 kiwi boni said:

That sugests there is a residual valve not letting pressure back into the resour Paul

What

Last Indian Said 🍻

its that master cylinder Paul !!!

now !what you need to do is..... pull the sucker out strip it down and find that tiny rubber residue valve !

Edited June 25, 2025Jun 25 by 64 kiwi boni